摘要:

140 MOSSBAUER SPECTROSCOPY OF 61Ni AUTHOR INDEX* * The references in heavy type indicate papers submitted for discussion. Bancroft G. M. 48 85 115. Bryukhanov V. A. 69. Bukshpan S. 132. Clark M. G. 19,49 101. Clear M. 13. Cordey Hayes M. 66 101 131. Danon J. 11 12 47 68 83. Davies D. W. 66. Duncan J. F. 13 15 47 64 65 83 99 103 115 116 118. Freeman A. G. 49. Gallagher P. K. 40 48 49 101. Gibb T. C. 99 117 131. Gol'danskii V. I. 59. Greenwood N. N. 15 20 29 39 47 48 51 Gutlich P. 84 97. Herber R. H. 19 86 97 98 99 100 101 132. Iofa B. Z. 69. Johnson C. E. 7 12 14 15 30. Kothekar V. 69. van der Kraan A. M. 38. 75 83 99 116 131. Krishnan R. 39 50. van Loef J. J. 38. Lyubutin I. S. 31. MacChesney J. B. 40. MacKede K. J. D. 103. Makarov E. F. 31 59. Parish R. V. 13 20 75. Pasternak M. 119 131 132.Perkins P. G. 51 65. 66 67 68. Pillinger W. L. 77. Povitskii V. A. 31. Semenov S. I. 69. Shpinel V. S. 69 75. Simopoulos A. 15. Spijkerman J. J. 134. Stewart D. J. 103 Stone A. J. 65. Stone J. A. 77 82 84 85 132. Trotter K. 13. Trozzolo A. M. 26. Wall D. H. 51. Wickman H. H. 21 29 30. 140 MOSSBAUER SPECTROSCOPY OF 61Ni AUTHOR INDEX* * The references in heavy type indicate papers submitted for discussion. Bancroft G. M. 48 85 115. Bryukhanov V. A. 69. Bukshpan S. 132. Clark M. G. 19,49 101. Clear M. 13. Cordey Hayes M. 66 101 131. Danon J. 11 12 47 68 83. Davies D. W. 66. Duncan J. F. 13 15 47 64 65 83 99 103 115 116 118. Freeman A. G. 49. Gallagher P. K. 40 48 49 101. Gibb T. C. 99 117 131. Gol'danskii V. I. 59. Greenwood N. N. 15 20 29 39 47 48 51 Gutlich P. 84 97. Herber R. H. 19 86 97 98 99 100 101 132. Iofa B. Z. 69. Johnson C. E. 7 12 14 15 30. Kothekar V. 69. van der Kraan A. M. 38. 75 83 99 116 131. Krishnan R. 39 50. van Loef J. J. 38. Lyubutin I. S. 31. MacChesney J. B. 40. MacKede K. J. D. 103. Makarov E. F. 31 59. Parish R. V. 13 20 75. Pasternak M. 119 131 132. Perkins P. G. 51 65. 66 67 68. Pillinger W. L. 77. Povitskii V. A. 31. Semenov S. I. 69. Shpinel V. S. 69 75. Simopoulos A. 15. Spijkerman J. J. 134. Stewart D. J. 103 Stone A. J. 65. Stone J. A. 77 82 84 85 132. Trotter K. 13. Trozzolo A. M. 26. Wall D. H. 51. Wickman H. H. 21 29 30.

ISSN:0430-0696    

DOI:10.1039/SF96701FX001

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

Hyperfine Interactions in Fe2+ Salts BY C. E. JOHNSON Solid State Physics Division Atomic Energy Research Establishment Harwell Berks. Received 20th September 1967 Magnetic hyperfine field data for a number of Fez+ salts are summarized and discussed in terms of the theory of hyperfine interactions in solids. The qualitative agreement between theory and experiment is good and the relative importance of the contributions from the orbital and spin moment of the ion seems to be understood. The effects of covalency are considered and it appears that if they could be quantiatively interpreted the measurement of hyperfine fields could be a powerful as well as sensitive method for studying chemical bonding. The Mossbauer effect has become a valuable tool for the investigation of magnetic ions in crystalline solids and many applications to the study of ferrous salts have been made.In this paper we summarize the data on a number of high-spin ferrous salts and show that in most cases they may be understood qualitatively on a simple theory. Measurements of hyperfine fields are useful in determining the spin part of the ground-state wave function of the ion and in principle if the interpretation could be made quantitative would yield information on the degree of covalency of the ion. Of the sources of energy shifts in Mossbauer spectra the isomer shift and the quadrupole splitting are commonly interpreted in terms of and correlated with the chemical properties of the ion. The isomer shift which gives a measure of the electron charge density at the nucleus is clearly related to the covalency.By means of calculations of the effect at the nucleus of changes in the outer electrons of the ion a detailed interpretation may be attempted. The quadrupole splitting arises from the asymmetry of the distribution of the electronic charge of the ion and its surroundings and is only indirectly related to the covalency of the ion. Its great value lies in providing a method for determining the orbital ground state of the ion in the crystal and for finding the direction of the axes of the crystalline electric field. The hyperfine field i.e. the magnetic field internal to the ion which acts on the nucleus is a complicated quantity which depends upon both the spin and orbital magnetism of the ion. It is a tensor which may be analyzed into anisotropic terms arising from orbital (IfL) and spin dipolar (Hd) fields and an isotropic term (Hs) which is proportional to the electron spin density at the nucleus.The latter is sensitive to the degree of covalency of the ligands as is known from studies of the spherically symmetric ions Mn2+ and Fe3+ (see table 1) where it is the dominant contribution. In Fe2+ salts the anisotropic terms are comparably large and so it would be necessary to be able to subtract these out quantitatively in order to study chemical bonding from hyperfine field data. Many ferrous salts are antiferromagnets and from the Mossbauer spectrum measured well below their Nee1 temperatures the component of the hyperfine field tensor H along the direction of spin alignment may be directly determined. 7 8 HYPERFINE INTERACTIONS IN Fe2+ SALTS For salts which remain paramagnetic down to liquid helium temperatures and which also have fast electron spin relaxation rates (which is generally true for Fe2+ ions) magnetic hyperfine splitting may only be observed if an external magnetic TABLE 1 .-HYPERFINE FIELD DATA FOR Fe3+ SALTS co-ordination Hn (kG) ref.FeF3 6 F- 622 a Fe2(S0,)3 . (NH&S04 . 24H20 6 H20 584 b Fez03 6 02- 540 C FeC13 6 C1- 487 d a D. N. E. Buchanan and G. K. Wertheim Bull. Amer. Physic. Soc. 1962 I1 7,227. b L. E. Campbell and S. DeBenedetti Physics Letters 1966 20 102. c T. Nakamura and S. Shimizu Bull. Inst. Chem. Res. Kyoto Univ. 1964 42 299. d C. W. Kocher Physics Letters A 1967 24 93. field H is applied in order to produce an appreciable magnetization. In practice this requires fields of about 30kG or more at liquid helium temperatures and allows the components Hn1 of the hyperfine interaction tensor along all the principal axis directions i to be determined.The effective magnetic field at the nucleus is then where ( S ) / S is the fractional magnetization produced which may be calculated from the applied field the temperature and a knowledge of the spin-Hamiltonian of the ion. Hence from the measured values of He, the components of H, may be deduced. Heff = H+ ( ( S ) I W m HYPERFINE FIELDS AND MAGNETIC PROPERTIES OF THE Fe2+ ION IN CRYSTALS The theory of hyperhe interactions in solids is based largely on the work of Abragam and P ~ y c e . ~ The hyperfine field is made up of several contributions These contributions and their relation to other measured properties of magnetic ions have been discussed by Marshall and by Marshall and J o h n s ~ n .~ using unrestricted Hartree-Fock wave functions. The value they obtained for the free Fe2+ ion is -550 kG. By comparison with the data for Mn2+ and Fe3+ this value may be reduced in an actual solid because of covalency. The core polarization field H has been calculated by Watson and Freeman The field due to the orbital moment is given by H L i = 4B(r-3Xgi-22) where gr is the component of the electron g-factor so that the orbital moment is (gf -2)S and S = 2 for Fe2+. r is the position of the electron relative to the nucleus and { ) denotes the average taken over the 3d electron wave function. The dipolar field arises from the asymmetry of the electron spin density and is therefore closely related to the quadrupole splitting which arises from the asymmetry of the electron charge density.In fact Hd = Pq where q is the electric field gradient and where I 0) is the orbital wave function of the ion. H~~ =~P(~-~>(O[L~-~L(L+~)IO), C. E. JOHNSON 9 Summing together all the contributions 1 H = H + 4 p y 3 ) (& -2) +-(0 L; -2 *}I [ l4 = H + 4p( r - 3)Ri where 1 14 Qi =(gi-2)4-(o~L~-2~o). The Fe2+ ion has a 3d6 . 5D configuration i.e. there is a single d-electron outside a spherically symmetrical half-filled shell. In a solid the orbital angular momentum is quenched by the crystal field and we denote the orbital ground state by I O} and the excited states by I n ) with energies An. Measurement of the sign of the electric field gradient from Mossbauer effect spectra enables the ground state 1 O} to be determined and the temperature variation of the quadrupole splitting allows the energies A to be e~timated.~ When 10) has been determined the dipolar field tensor may be calculated.The fivefold spin degeneracy of the orbital ground-state is removed by the spin- orbit coupling acting together with the crystal field. This also partly restores some of the orbital moment. As an illustration we consider the case where the symmetry is high enough so that the only orbitals populated are dxy dvz and dzx and we assume that dxy is lowest in energy. This corresponds closely to the situation in several of the salts we shall discuss. Then the g-factors are where A is the spin-orbit coupling parameter (A = -104cm-l for the free Fez+ ion). In many salts the g-factors are known from magnetic susceptibility measure- ments.In others they may be estimated from the values of the A, deduced from the temperature variation of the quadrupole splitting. If the splittings are large compared with kT the electric field gradient at a temperature T is approximately whence the A the g-values and the orbital hyperfine fields may be calculated. In the special case where one orbital say dzx is much higher in energy than the other two the susceptibility has axial symmetry about the x-axis with g = gz = 2 and There are errors from e.g. the neglect of thermal expansion but in the absence of anything better this approach can be useful. INTERPRETATION OF HYPERFINE FIELD DATA FOR Fez+ SALTS Table 2 summarizes an analysis of the hyperfine fields measured in several Fe2f salts using the above simple theory.Two of the crystals FeSiF6. 6H20 and FeCl . 4H20 are paramagnetic at 4.2"K and the data were taken in an external magnetic field and the remainder are antiferromagnetic. The g-values were taken from susceptibility measurements except for FeF which has been studied by C. E. JOHNSON 9 Summing together all the contributions 1 H = H + 4 p y 3 ) (& -2) +-(0 L; -2 *}I [ l4 = H + 4p( r - 3)Ri where 1 14 Qi =(gi-2)4-(o~L~-2~o). The Fe2+ ion has a 3d6 . 5D configuration i.e. there is a single d-electron outside a spherically symmetrical half-filled shell. In a solid the orbital angular momentum is quenched by the crystal field and we denote the orbital ground state by I O} and the excited states by I n ) with energies An. Measurement of the sign of the electric field gradient from Mossbauer effect spectra enables the ground state 1 O} to be determined and the temperature variation of the quadrupole splitting allows the energies A to be e~timated.~ When 10) has been determined the dipolar field tensor may be calculated.The fivefold spin degeneracy of the orbital ground-state is removed by the spin- orbit coupling acting together with the crystal field. This also partly restores some of the orbital moment. As an illustration we consider the case where the symmetry is high enough so that the only orbitals populated are dxy dvz and dzx and we assume that dxy is lowest in energy. This corresponds closely to the situation in several of the salts we shall discuss. Then the g-factors are where A is the spin-orbit coupling parameter (A = -104cm-l for the free Fez+ ion).In many salts the g-factors are known from magnetic susceptibility measure- ments. In others they may be estimated from the values of the A, deduced from the temperature variation of the quadrupole splitting. If the splittings are large compared with kT the electric field gradient at a temperature T is approximately whence the A the g-values and the orbital hyperfine fields may be calculated. In the special case where one orbital say dzx is much higher in energy than the other two the susceptibility has axial symmetry about the x-axis with g = gz = 2 and There are errors from e.g. the neglect of thermal expansion but in the absence of anything better this approach can be useful. INTERPRETATION OF HYPERFINE FIELD DATA FOR Fez+ SALTS Table 2 summarizes an analysis of the hyperfine fields measured in several Fe2f salts using the above simple theory.Two of the crystals FeSiF6. 6H20 and FeCl . 4H20 are paramagnetic at 4.2"K and the data were taken in an external magnetic field and the remainder are antiferromagnetic. The g-values were taken from susceptibility measurements except for FeF which has been studied by C. E. JOHNSON 11 which confirms the basic correctness of the understanding of the orbital and dipolar fields. The anisotropy in the hyperfine field tensor in the paramagnetic crystals FeSiFs . 6H20 and FeCl . 4H20 is well accounted for. The approximate linear law found in fig. 1 shows that (r3) and H for the salts considered which are all hydrates do not vary greatly. For a qualitative interpretation of the data it would be necessary to take account of covalency which would be expected to reduce both 1 H J and {r3).The latter is associated with the observed reduction of the spin- orbit coupling parameter For salts which are considerably more covalent there is evidence for a greatly reduced isotropic hyperfine interaction. For example in (NMe,)FeCl where a Fe2+ ion is tetrahedrally co-ordinated to four chlorine ions the hyperfine field observed is only -38 kG,ll and in FeCl with co-ordination to six chlorine it is almost zero.12 Both these salts have quadrupole splittings which indicate that (r3) has not been drastically reduced. They also have large orbital fields but to get such small hypcrfine fields I H I must have been reduced perhaps to about 350 kG. It is evident that a systematic study of hyperfine fields in the more covalent Fe2+ compounds should be valuable in providing information on the effects of chemical bonding.As a method for investigating covalency they could if correctly inter- preted by more useful than isomer shift data as they are more sensitive to small changes . and has been discussed by several authors.g* lo L. R. Walker G. K. Wertheim and V. Jaccarino Physic. Rev. Letters 1961 6 98. J. S. van Wierengen Disc. Furaday Soc. 1955 19 118. A. Abragam and M. H. L. Pryce Proc. Roy. SOC. A 1951 205 135. W. Marshall Physic. Reu. 1958 110 1280. W. Marshall and C. E. Johnson J. Physique Rad. 1962,23 733. R. E. Watson and A. J. Freeman Physic. Rev. 1961 123 2027. ' R. Ingalls Physic. Rev. A 1964 133 787. * J. Owen Proc. Roy. Soc. A 1955 227 183. W. Marshall and R. Stuart Physic. Rev. 1961 123 2048. lo R. G. Shulman and S . Sugano Physic. Rev. 1963,130,517. P. R. Edwards R. J. P. Williams and C . E. Johnson J. Chem. Physics 1967,147 2074. l 2 K. Ono and A. Ito J. Physic. SOC. Japan 1964 19 899.

ISSN:0430-0696    

DOI:10.1039/SF9670100007

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

C. E. JOHNSON 11 GENERAL DISCUSSION Prof. J. Danon (Riu de Janeiru) said Accurate measurements of isomer shifts show that this hyperfine interaction is sensitive to small changes arising from chemical bonding. Precisely with the iron (11) chlorides it has been found a systematic variation of the isomer shift with the hydration number.l These results indicate that the degree of covalency decreases with the increase in the number of coordinated water molecules around the iron atom. One of the difficulties for interpreting the hyperfine fields in terms of covalency has been pointed out by van Wieringen with the Mn2+ salts a small mixture of excited states involving s-shells such as (3s-4~) gives a very large contribution to the hyperfine field. The temperature variation of the electric field gradient as used in Johnson’s treatment is not sensitive to the presence of such states which involve s-electrons.J. W. Hurley Jr. R. C. Axtmann and Y. Hazony Bull. Amer. Physic. SOC. 1967 12 654. Dim. Faraday SOC. 1955 19 118. 12 GENERAL DISCUSSION Dr. C. E. Johnson (A.E.R.E. Harwell) said In reply to Danon the range of variation of the values of the hyperhe field with chemical bonding is in general larger than that of the isomer shift. It is in this sense that hyperfine field measurements are more sensitive. Regarding your second point the energy of the excited atomic states is surely too high to be affected directly by chemical bonding. One can therefore qualitatively associate a change in hyperfine field of nearly S-state ions with a change in d-electron spin density produced by covalency.Prof. J. Danon (Rio de Janeiro) said The following electronic mechanisms can affect the isomer shift of Fe5' in its compounds 1-3 (a) direct contribution from 4s bonding ; (b) indirect contribution from 3d bonding. From free-ion wave functions an increase in the number of 3d electrons increases the shielding of s electrons (mainly 3s) and thus decreases their density of the nucleus. Changes in 3dpopulation by bonding could have the same effect on the s-electron density at the nucleus. This can occur in two ways (i) covalency between d electrons and filed ligand orbitals which will increase the population of d electrons on the metal ; (ii) the bonding of d electrons with empty ligand orbitals will decrease the d-electron density at the metal ion because of back-donation.There is however some evidence suggesting that mechanisms (i) i.e. donation from the ligands to the 3d orbitals may not be an important one for determining the isomer shift in high-spin iron c~mplexes.~ TABLE 1. 3d 4s I.S. (cm/sec) Fe"C1 7.01 0.27 + 0.12 Fe"'C1 6.92 0.27 + 0.04 Table 1 lists the M.O. populations recently calculated for the ferrous and ferric tetrachlorocomplexes and the respective isomer shifts referred to a Cr source. The difference in isomer shift which is of similar magnitude as that observed with other + 2 and + 3 high-spin iron complexes cannot be explained on the basis of the M.O. populations. This can be interpreted by considering that the shielding of the 3s orbitals by the 3d orbitals may not be altered by the increase in the 3d orbital population due to bonding.A possible reason for this is the following as a con- sequence of the increase in the 3s population a spread of the 3d wave function occurs. It is known (nephelauxetic effect) that the average radius of the 3d shell is larger in bonded transition ions as compared to the free ion. From the point of view of the shielding of the 3s orbitals the increase in 3d population may be compensated by the spreading of this orbital. Evidence for this type of compensation mechanism has been recently discussed and it has been shown that reduced charge on the metal due to increased covalency compensates for most of the natural contraction of the 3d radial function with increasing nuclear charge. As a consequence the shielding of the 3s by the 3d orbitals remain unaffected by ligand-to-metal 3d bonding and its value is that corresponding to the free-ion configurations.When empty ligand orbitals are available for back-donation the spreading of the 3d charge will occur without any compensating increase in its value. In this situation the 3s shielding is markedly affected by the expansion of the 3d wave functions as has been shown with the complex ion cyanides and nitrosyl~.~* L. R. Walker G. K. Wertheim and S. Jaccarino Physic. Rev. Letters 1961 6 18. R. G. Shulmann and S. Sugano J. Chem. Physics 1965,42,39. J. Danon in Applications of Mossbaiier Efect in Chemistry and Solid State Physics (I.A.E.A. Vienna 1966) p. 89. H. Bash A. Viste and H. B. Gray J. Chern. Physics 1966 44 10. J. Danon and L. Iannarella J. Chem. Physics 1967 47 382. GENERAL DISCUSSION 13 Dr.R. V. Parish (University of Munchester Inst. of Sci. and Technology) said Danon suggests that the observed difference in isomer shift between FeC14 and FeCla- is better accounted for using the free ion 3d-populations (3d5 and 3d6) than those obtained by molecular orbital methods. The latter figures seem unreliable at present \ \ \ \ \ \ \\ \\ \ / / \ aln - as judged by the variety of figures available. I should like to suggest an additional factor which does not seem to have been taken into account and which may possibly explain why the effects of donation from the ligands to the 3d-orbitals may not be as large as one imagines. Consider the molecular orbital scheme for an octahedral complex. The bonding molecular orbitals represent the effects of ligand to metal donation.Since these orbitals we are told have some 10-20 % 3dcharacter to this extent we have an increase in the 3d-population over the free ion value. In a rough way one may say that the d-character of the bonding e orbitals is compensated by a corresponding degree of ligand character in the ez antibonding orbitals i.e. electrons in the e* orbitals are delocalized back to the ligands in a type of metal to ligand a-bacl- donation. In high-spin complexes there are two eg electrons both for ferrous and ferric complexes. Thus the effect of donating four electrons into the 3d,e orbitals is partially (roughly half) offset by back-donation of two electrons in e orbitals. Finally a treatment of this kind is much more difficult for tetrahedral complexes since in this case there is considerable mixing of 3d and 4p orbitals.Mi. M. Clear Prof. J. F. Duncan and Mi-.K.Trotter (Victoria University of Welling- fotz N.Z.) said A question was raised as to how covalency effects might be allowed for or investigated in relation to hyperhe fields. It was suggested that the effects of covalency could be more effectively investigated by application of external fields to compounds which are known to be covalent and these could then be compared with those which are not. The extent to which the nuclear hyperfine field is detectable depends on the relaxation time and the temperature. The observed field will also depend on the magnitude of the applied field. Even at room temperature the line width increases by 5-10 fold with applied fields of 25 kgauss depending on the spin 14 GENERAL DISCUSSION type.With high spin Fe2+ the increase is rather smaller than for low spin Fe" (e.g. [Fe(CN),]"-) even after making allowance for the quadrupole interactions of the former. The covalency present in low spin Fe" eliminates the spin and orbital contributions present in high spin Fe2+ so that the following features become apparent. (a) An estimate of the degree of covalency present in compounds of unknown electronic structure can be obtained by measuring the line broadening. This would be expected to be more sensitive at lower temperatures where hyperfine fields could be directly measured. (b) The hyperfine field is affected by the external field by at least two mechanisms as is evident from the relatively different effects of high and low external fields. Undoubtedly direct spin-field interactions are important but it seems likely that spin polarization plays a significant role as well.In view of the intense magnetic fields present in atoms one may well ask whether these play any role in banding (even though magnetic interactions are relatively weak compared to electrostatic inter- actions). An interesting case is iron (111) ammonium alum in which the line width is ob- served to decrease at about 2 kgauss external field but to increase as expected at higher fields. This does not occur however if the iron (111) is replaced by aluminium (111) ions until it is present in about 3 mole % only. Spin-spin or spin-lattice interactions are therefore apparently occurring which is equivalent to regarding the pure 5S state of the iron (111) atom as mixing with a higher energy state-i.e.covalency effects are present. The energy difference between the two states must however be small since the spectra are not detectably different at low fields when the absorber is cooled in liquid nitrogen. Dr. C. E. Johnson (A. E. R. E. Harwell) said In reply to the remarks of Duncan et al. I am not clear as to their meaning. The core polarization field H and the effective value of (r3) are both sensitive to covalency. But measurements 011 paramagnetic ions (e.g. Fe3+) in external magnetic fields at room temperature are not simple to interpret as the spectra are dominated by relaxation effects. The broadening cannot necessarily be described in terms of an effective magnetic field at the nucleus but may depend upon all the components of the magnetic hyperfine interaction tensor.To extract the values of these components (and hence H and (r3)) from the data is not straightforward and I would doubt that their conclusions about the determina- tion of the degree of covalency could be generally valid. The complexity of the behaviour of Fe3+ in the alums has been extensively studied by Housley and de Waardl and by Campbell and de BenedettL2 In low spin (i.e. S = 0) Fe" compounds one would expect no internal magnetic field at the nucleus and the broadening should be due solely to the external field at all temperatures. This is also true for high spin Fe2+ at room temperature and is shown in the spectra measured by Grant et aL3 So I do not understand your state- ment that high spin Fe2+ broadens less than low spin Fe" in a magnetic field.Regarding the role of these intense magnetic fields in chemical bonding it must be remembered that the core polarization field H is not a real magnetic field but is a convenient way of expressing the " contact " interaction between s-electrons and the nucleus. This field therefore only has a meaning at the nucleus and although its magnitude is affected by covalency it cannot be said to make a contribution to chemi- cal binding. Housley and H. de Waard Physics Letters 1966 21 90. Campbell and de Benedetti Physics Letters 1966 20 102. Grant et al. J. Chem. Physics 1966 45 1015. GENERAL DISCUSSION 15 Prof. J. I?. Duncan (Victoria University of Wellington N.Z.) said In reply to Johnson I agree with your remarks in general which put the situation very clearly but your reply makes it desirable that I should clarify a few aspects of our work.(a) The internal field observed with FeI1 has approximately the same slope at high fields as that predicted for a bare nucleus but up to about 4 kgauss the lines hardly seem to broaden at all. It is possible that this is an experimental artefact but we believe not. (b) In our work broadening is usually observed Fe" > Fe3+. Fez+ has a quadrupole interaction and therefore a correction is necessary to allow for this different feature. Fez+ then falls between Fe" and Fe3+. Prof. N. N. Greenwood (Newcastle upon Tyne) said I would ask Johnson whether his analysis for high spin d6 (four unpaired electrons) could be applied directly to high spin c14 (e.g. F P ) ; and if so what changes would be necessary. In Gallagher's paper the hyperfine field at FcIV in a perovskite is shown to be about 270 kOe which is similar to that in several of Jolinson's FeI1 compounds.I am not sure that I followed all the points made by Duncan but one point of principle bothers me. The single line is shown as splitting into six separate peaks at 0.46 0.36 0.28 0.20 0.1 1 and 0.00 nim sec-I. The separation between these peaks is considerably less than the Heisenberg natural line width for an ideal 57Fe source- absorber pair and they could not therefore be resolved even if the experimental line width approached the natural line width. Prof. J. F. Duncan (Victoria University of Wellington N.Z.) said The results on one compound viz. iron (111) ammonium sulphate were chosen to illustrate the kind which could be obtained. No claim was made that six lines were resolved indeed the slide showed that they were not.However there was certainly a systematic fluctua- tion in the observed intensity in the trough of the resonance which seemed to indicate the presence of separate resonances approximating to the values quoted by Green- wood. In the example shown the variation in intensity was of the order of statistical fluctuation but cases have been observed by us with identifiable peaks some five times larger than the standard deviation of the statistical fluctuation in magnetically lip-e-broadened Mossbauer spectra. One does not necessarily expect six lines to be observable in such spectra because in addition to the difficulty of separating the nuclear hyperfine peaks there is also the possibility of crystal field and spin-spin interactions which could affect the resonance energy.However in my comments on the spectra shown the significance of the resonance intensity was not discussed. The important feature was the line width which is cleaxly evident as broadening enor- inously on application of a magnetic field in a way which depends on the spin type of the species concerned. Dr. C. E. Johnson (A. E. R. E. Hawell) said In reply to Greenwood the two ions are similar ; high spin d6 may be considered as an electron outside a spherical d5 core while high spin d4 is a hole in the half-filled d-shell. FexV therefore has a spin orbit coupling parameter of opposite sign (i.e. positive). Dr. A. Simopsulos (Greek At. Energy Comm. Athens) said I would like to present some data on paramagnetic relaxation of Fe3+ ions.Samples of 57Fe impurities bound in LiOH and Ca(OH) were used as sources. Single line absorbers This work was carried out at the Soreq Nuclear Research Center Yavne Israel in collaboration with I. Pelah and P. Hillman. 16 GENERAL DISCUSSION of sodium ferrocyanide were used in order to detect the hyperhe splitting and the absolute effect in the sources. Mossbauer experiments were performed in the temperature range 20-300°K. Fig. 1 shows the temperature dependence of the LiOH ; Fe57 spectra. From the andysis these spectra consist of two parts one due to 57Fe in Co aggregates formed in the sample and the second due to paramagnetic hyperhe interaction of the Fe3+ ions. The shape of this second part is shown in fig. 2 where -0 - 2 - 4 - 6 x 0- x - .2 2- 8 2 % 4- E c) 5- - 2 - 4 -5 ii 222 O K +I0 +8 +6 +4 +2 0 -2 -4 -6 -8 -10 I I I I I I L - U velocity (mmlsec) FIG.1 .-The effect of temperature on the relaxation time of the LiOH Fe5’ polycrystalline source. The absorption is corrected for background. Absorption scales for each spectrum are shown next to the corresponding temperature. we have subtracted the part due to Co aggregates from the total spctrum. The temperature dependence of the spectra is typical of paramagnetic relaxation of the Fe3+ i0n.l Hyperfhe spectra due to the I & 5/2) spin state are clearly shown in the temperature range 20-200°K. The I f3/2) spin state displays hyperfine spectra around 20°K. The I f 1/2) spin state has too short a relaxation time to display any magnetic spectra in the temperature range examined but its effect is apparent from the broadening of the corresponding unsplit line at lower temperatures.The spin H. H. Wickman Mussbauer Methodology (Plennum Press 1966) vol. 2. GENERAL DISCUSSION 17 lattice relaxation time z was calculated from the broadening of the lines. The follow- ing temperature dependence fits these data with This temperature dependence is characteristic of Raman processes for impurities participating in a localized mode wo = kO/h The measured mode at 8 = 280°K is r1 = AT2 +B exp (- OiT) A = 400 sec-l deg.-2 ,B = lo7 sec-' and 8 = 280°K. -- 273 "K 0.9 m rn / s t c V E = 4% 235 "K IJ 1 I +I0 +8 +5 +4 +2 0 -2 -4 -6 -8 -10 velocity (minlsec) FIG. 2.-Mossbauer spectra of fig. 1 after subtraction of the Co aggregates part. The hyperfine structure for the I &5/2> state is shown by the broken line and along the bottom of the figure.J. Murphy Physic. Reu. 1966 145 241. 18 GENERAL DISCUSSION in agreement with optical modes of Li0H.I The crystal field splitting of Fe3+ was determined from depopulation effects of the I t5/2> spin state and was found to be D = 1.6 x eV. The Ca(OH) source displayed magnetic hyperfine spectra in the temperature range 20-300°K (fig. 3). Three different iron sites were assigned to these spectra. I I I 1 1 I I 4l 6 I 2 I 4 t G' G -235°K I t f ! I t 4 f 26OK GI C D - I I I I I I 1 +I0 +8 +6 +4 +2 0 -2 -4 -6 -8 -10 velocity (mmlsec) FIG. 3.-Mossbauer spectra of FeS7 in The absorption is corrected for background. The broken line shows the shape of the central part of the spectra after subtraction of the magnetic peaks corresponding to the I 5/2) state.Two of them are trivalent iron ions and the third is divalent iron. This complexity of the spectra did not permit a quantitative study of the temperature dependence of the spin-lattice relaxation time. However comparison with the spectra of the LiOH source shows that the relaxation time of the trivalent iron sites has a temperature dependence even weaker than that displayed by the iron site of the LiOH source. This is due to the fact that the energy of the optical modes of Ca(OH) is higher than that of Li0H.l The divalent iron displays only a quadrupole splitting. Due to the strong spin-orbit coupling the spin-lattice reIaxation of this ion is too short to give rise to magnetic hyperfine spectra. I. Pelah K. Krebs and Y. Imry J.Chem. Physics 1965,43 1864. GENERAL DISCUSSION 19 Mr. M. G. Clark (University of Cambridge) (communicated) In connection with the remark by Simopoulos the rapid spin-lattice relaxation of high-spin Fe2+ is in fact due to the orbital angular momentum of the corresponding free-ion term being non-zero. Thus there is strong orbit-lattice coupling which leads to rapid transi- tions. This point is discussed in more detail in a study of the role of electronic factors in the paramagnetic relaxation of high-spin Fe2+. It has also been shown that partial quenching of the orbit-lattice interaction of high-spin Fe2+ may sometimes occur e.g. in a square-planar envir0nment.l. Prof. R. H . Herber (Rutgers-The State Uniuersity) said I would like to make reference very briefly to two very recent papers from Drickamer’s group at Illinois and an ingenious idea first proposed to me by Dr.Y. Hazony at Princeton University which ties Drickamer’s work to the present discussion concerning 57C0 “ probe ” atoms in Mossbauer spectroscopy. What Drickamer and his colleagues have done is to subject a number of iron complexes (used as Mossbauer absorbers) to pressures up to -200 kbar and study the pressure dependence of the I.S. and Q.S. parameters. The results are quite dramatic especially the ferric complexes. In K,Fe(CN) 6 for example the isomer shift becomes more negative as the pressure is increased and at about 50 kbar suddenly becomes more positive by about 1*2mm/sec. Then as the pressure is further increased the isomer shift again decreases slowly almost to its original value at about 175 kbar.A similar “jump ” is observed in the pressure dependence of the quadrupole splitting. The authors identify the high-pressure form with ferrous iron-on the basis of its IS. and Q.S. parameters- which is formed reversibly under pressure. In a parallel study of Fe2(S04), FeP04 and ferric acetyl acetonate inter alia they find typically about 50 % of the iron in Fe2+ states at 150-200 kbar. These results may be related to what is observed in the Mossbauer spectra of 57C0 compounds (not doped !) where one sees in spectra produced with Co3+ sources resonance peaks which can be ascribed to Fe2+ Fe3+ and higher positive charge states. The latter are accounted for-at least qualitatively- by invoking an internal conver- sion-Auger effect cascade and a number of calculations dealing with the formation of such states have appeared in the literature.What has always been a mystery is how to account for charge states lower than those which obtain in the parent coin- pound and which are clearly seen in cobalticinium salts,4 cobalt (111) acetylacetonate and in the elegant work done recently by Sano in our laboratory on cobalt (111) cyanides. Hazony’s idea is the following in typical ionic compounds the ionic radius of Co3+ is slightly smaller than that of Fe3+. Therefore an iron atom in the 3+ charge state formed by the ex. decay of a 57C0 atom sitting in a Co3+ lattice site sits in a cavity which is slightly too small to contain it. In other words it experiences an effective pressure. We have done a few model calculations using compressibility data for typical ionic transition metal compounds where such information is available (e.g.for Fe20, the constant p = -(l/Vo)(aV/ap)T is 6 . 0 ~ lo-’ and for FeCO, /? = 1-0 x and the effective “ internal pressure ” which one calculates is 70-100 kbar. It is thus possible considering the data of Drickamer et al. that one can account M. G. Clark J. Chein. Physics 1968 in press. M. G. Clark G. M. Bancroft and A. J. Stone J. Chem. Physics 1967,47,4250. A. R. Champion R. W. Vaughan and H. G. Drickamer J. Chem. Physics 1967,47,2591. G. K. Wertheim and R. H. Herber J. Chem. Physics 1963,38 2106. G. K. Wertheim W. R. Kingston and R. H. Herber J. Chem. Physics 1962 37 687. H. Sano and R. H. Herber to be published. 20 GENERAL DISCUSSION for the apparent presence of Fe2+ in a Co3+ matrix by postulating such a mechanism Whether or not this explanation will prove to be the correct one cannot be predicted but the idea seemed an interesting one which merits further experimental testing and discussion.Finally possibly one bonus of the Hazony-Drickamer idea is that it would account neatly for the observation that anomalous charge states have never been observed in the 119Sn isomeric transition decay although we have looked for such effects-for which the a priori expectation is very high due to the large internal conversion co- efficient of the 65 keV transition-for many years. Experiments both at liquid nitrogen and at room temperature with a wide variety of ionic covalent Sn2+ and Sn4+ sources have always been negative.l Dr. R. V. Parish (University of Manchester Inst. of Science and Technology) said Would Herber inform me whether there are any data for low-spin cobaIt(II1) com- plexes decaying to high spin iron(II1) complexes? This would be a good system to investigate since the change in ionic radii will be large.Prof. N. N. Greenwood (Newcastle upon Tyne) said Three items puzzle me about Herber’s ingenious explanation of the pressure induced simulation of a ferrous chemi- cal shift by a ferric ion (i) the pressure would relax within the time available (- lo-’ sec) since an acoustic wave traverses the dimensions of one unit cell in a crystal within about 10-l2 sec; (ii) the influence of cationic polarization on the precise position of the contiguous anions has been neglected in calculating the pressure Fe3+ is likely to be less polarizing than Co3+ and this would again tend to reduce the calculated pressure ; (iii) in non-cubic environments high spin Fe2+ can readily be distinguished from Fe3+ by its much larger quadrupole interactions. Is it not possible for the iron ion to reach its preferred oxidation state in the lattice by the same process that is involved in replacing the large number of electrons which ‘‘ boil off” from the ion in the cascade process which follows the nuclear electron capture event which transmutes 57C0 into 57Fe? * l €3. Yoshida and R. H. Herber unpublished results. * see also comments by Gallagher and by Herber p. 101-2.

ISSN:0430-0696    

DOI:10.1039/SF9670100011

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

Mossbauer Studies in Spin 3/2 Iron Complexes BY H. H. WICKMAN AND A. M. TROZZOLO Bell Telephone Laboratories Incorporated Murray Hill New Jersey Received 16th October 1967 A series of penta-coordinate bis(N,N-dialkyldithiocarbamato) iron (111) halides have been prepared in which the ground iron (111) term is an orbital singlet and spin quartet. The quartet ground term results from the low symmetry (C2”) local environment of the iron. The Mossbauer technique was employed to study several homologues closely related to the basic unit bis(N,N-diethyldithiocarbamato) iron (111) chloride which exhibits a ferromagnetic ordering at low temperatures (T‘ = 25°K). While the paramagnetic character of the ground quartet remains independent of small structural changes no new magnetically ordered systems were found.However low-temperature Mossbauer data show that the small zero-field splitting of the quartet manifold is markedly affected by the halo-constituent of the penta-coordinate complex. Effects of a small external polarizing field on the Mossbauer spectra of these complexes is also considered. A number of recent investigations of the bis(N,N-dialkyldithiocarbamato) iron (111) halides have shown that the ground electronic term of the trivalent iron is an orbital singlet and spin Martin White and Hoskins have reported FIG. 1.-Structural formula and local iron sym- metry in Fe(S2CNEt2)2CI after Hoskins Martin and White.’ 81s (N N - DIETHYLDIT~(IOCARBAMATO) IRON~CHLORIDE \ C 2 7 I I 2 2 6 the crystal structure of the diethyl-chloro derivative together with room and nitrogen temperature susceptibility and e.p.r.data for several homologues. The structural formula and local iron environment of the Fe(S2CNEt2),Cl complex are shown in fig. 1. The chemical similarities of all of the related complexes prepared to date 21 22 SPIN 3/2 IRON COMPLEXES indicate that the iron environment is qualitatively similar in different species.,Y The point group symmetry at the iron (111) is therefore assumed to be CZv and the ligand fields have rhombic symmetry. A magnetic investigation of the diethyl-chloro derivative has shown a ferro- magnetic ordering of the iron at 2.5"K3 Single crystal e.p.r. measurements within one of the Kramers levels of the S = 3/2 multiplet of the paramagnetic diisopropyl-chloro complex have shown the zero field splitting of the two Kramers levels to be approximately 5 cm-I.The later data are consisent with early Mossbauer studies of relaxation effects in this complex.6 In the following synthetic methods were employed to obtain several compounds representing the minimal structural changes that are possible starting with the basic unit Fe(S,CNEt,),Cl. A methyl group has been added to or subtracted from the alkyl constituent and a bromide ion has replaced the chloro group. Bulk d.c. and a.c. susceptibility data shown that with any of these substitutions the magnetic ordering property is lost. However the paramagnetic character of the ground quartet remains substantially independent of structural changes. The difference in magnetic properties among the compounds appears as a change in sign of the splitting of the lowest quartet level on substitution of a bromo for a chloro group ; low-temperature Mossbauer effect (ME) data illustrate this effect.The effect of an external magnetic field on the Mossbauer patterns in these complexes is also investigated. LO W-TEMPERATURE MOSSBAUER EXPERIMENTS The Mossbauer effect in the four derivatives mentioned above has been observed with sample temperatures from 300 to 1.2%. At temperatures in the range 2-10°K complex relaxation effects have been observed in the chloro derivatives;3* a full discussion of these spectra will be given elsewhere. With the exception of Fe(S,CNEt,),CI the samples showed no magnetic orderings. These results were also confirmed by d.c. susceptibility data to 1.4"K and by a.c. susceptibility data to 0.3"K. The four homologues have effective moments derived from d.c.suscepti- bility data which show no marked dissimilarities and are in good agreement with the theoretical effective moment of 3-88 B.M. predicted for S = 3/2 and g = 2 ~ 0 0 . ~ Information concerning the magnetic hyperfine interactions within the quartet level is best derived from low-temperature data where relaxation times z are often long enough to allow a well-resolved paramagnetic h.f.s. In fig. 2 we give the Mossbauer absorption at 1 *2"K of (a) Fe(S2CNEt2)2Br (b) Fe(S,CN(i-propyl),),Cl (c) Fe(S2CNEt2)2Cl and (d) Fe(S,CNMe,),Cl. The nuclear parameters consistent with these data may be found in table 1. The prominant features of the spectra are summarized as follows. (a) Fe(S2CNEt,),Br. A simple quadrupole doublet is observed from 300 to 1.2"K. No appreciable temperature dependence was found ; at 1~2°K the splitting is 0.288 0.004 cmlsec.(b) Fe(S2CN(i-propyl)2)2C1. A typical multiline pattern is found. The data are consistent with a paramagnetic hyperfine field He, together with a large electric field gradient (EFG) with major axis perpendicular to the direction of He,,. In table 1 8 and 4 are conventional spherical polar co-ordinates defining the orientation of the EFG tensor with respect to He,,. (c) Fe(S,CNEt,),Cl. This spectrum is similar to that of (b). In the latter case however the sample is paramagnetic while here the material is ferromagnetically ordered T = 2.5"K. The parameters characterizing the data are given in table 1 ; 23 the similarity of these parameters with those of the paramagnetic homologue (b) is striking.The data reported here give the polycrystalline absorption in the ordered state at 1.2"K. In the earlier work the absorber was composed of larger crystallites H H. WICKMAN AND A. M. TROZZOLO FIG. 2.-Mossbauer effect at 1-2"K in poly- crystalline absorbers of (a) Fe(SzCNEt2)2Br (6) Fe( S2 CN( i - p r ~ p y l ) ~ ) ~ CI (c) Fe( S2CNEt 2)Z C1 and (d) Fe(SzCNMe2)2Cl. 1 I t " ' I ' ' I ' -0'8 0.6 -0'4 - 0 2 0 0 ' 2 0 ' 4 3.6 0.8 cm/sec TABLE 1 .-NUCLEAR SPIN-HAMILTONIAN PARAMETERS WHICH REPRODUCE THE MOSS BAUER SPECTRA OF FIG. 2 complex nuclear hyperfine data (a) Fe(SzCNEt2)zBr AE(1.2"K) = 0.288 f0.004 cm/sec = e q ~ / 2 4 1 +v2i3 (b) Fe( S 2CN( i-pr~pyl)~) ,C1 Heff = 334 f5 kOe eqQ/2 = 0.268 f0-003 cmlsec. y = 0*16f0.01 4 = 0 0 = 90". ( c ) Fe( S CNEt 2 ) C1 He€€ = 333 f 5 kOe eq&/2 =0*268 f0.004 cmlsec = 0.15 f o .o i 4 = 0 e = goo. z 0.15,4 = 0 e = goo. (d) Fe( S &NMe2) 2Cl Heff = 338 &lo kOe ; eqQ/2 E 0.266 cmlsec. which have a tendency to pack in such a way that a geometrically polarized absorber was produced. The relative intensities of fig. 2(c) and those in ref. (3) fig. 4 are slightly different for this reason. ( d ) Fe(S2CNMe,),Cl. In this system a fairly well-defined magnetic h.f.s. is observed. The material is paramagnetic and the broadening of the lines arises from electronic relaxation among the electronic levels. The estimated field If,,, 24 SPIN 3 / 2 IRON COMPLEXES and EFG are similar to those in (b) and (c). The shorter relaxation times are probably mainly due to decreased iron-iron separations in this compound which is the smallest homologue studied.A small transverse (with respect to the pray directions) polarizing field of 7 kOe was applied to the absorbers in the helium temperature range. The effect on the paramagnetic chloro-derivatives was generally small for temperatures where well resolved magnetic 1i.f.s. was observed (7 % o; I). For regions of intermediate relaxation times (z,-o;~) the effect of the field was to decrease relaxation times. However no striking features (aside from relative intensity changes and apparent line broadening occurring because polycrystalline absorbers were used) developed in the spectra of these derivatives. The effect in the ferromagnetic derivatives is more complicated and will not be discussed in detail here. 95 t L 9d I I I I I J 1 -4 -3 -2 -4 0 'I a 2 *3 -4 cmlsec FIG.3.-Mossbauer effect in Fe(S2CNEt2)Br with an external polarizing field of 7 kOe. In the di-ethyl-bromo derivative an interesting pattern was observed at 3-O"K and is shown in fig. 3. Similar patterns were found at higher temperatures but the doublet triplet character was most pronounced at about 4°K. This type of behaviour is indicative of the combined effect of a large randomly-oriented magnetic hyperfine field together with a quadrupole interaction. DISCUSSION The ground electronic term in the present series of complexes is an orbital singlet and spin quartet. The magnetic properties of the iron ion are therefore to good approximation ascribed entirely to the four levels of the S = 3/2 manifold. The interpretation of the Mossbauer magnetic h.f.s. requires knowledge of the magnetic interaction between the quartet levels and nucleus.In orbitally non-degenerate iron states such as Fe3+ 6S or the present case the interaction with the nucleus is primarily due to the isotropic contact interaction arising from core polarization. The standard procedure under these circumstances is first to characterize the electronic levels with a spin Hamiltonian S = 3/2 and then perturb these levels with the isotropic hyperfine interaction. The spin Hamiltonian parameters for one of the paramagnetic homologues have been determined from e.p.r. data and we begin by summarizing these results. H. H. WICKMAN AND A. M. TROZZOLO 25 Single crystal e.p.r. data have been reported for the Fe(S,CN(i-propyl),),Cl deri~ative.~ These results may be described by the spin Hamiltonian appropriate to rhombic symmetry with S = 3/2 and g = 2.00.The experimentally determined parameters D and iZ=E/D were 4.0°+0.5"K and 0.036+0*003 respectively. In the absence of an external field the quartet manifold is split into two Kramers doublets spaced by I 2 0 J m I . For simplicity? we assume that A = 0 (i.e.? E = O) so that the two doublets are I Ms = &3/2) and I Ms = 1/2). When D is negative the I Ms = &3/2) level is lower lying and positive D reverses the ordering? as shown in fig. 4. At low temperature 2% 1°K < I 2 0 1 only the ground doublet will be responsible for the Mossbauer magnetic h.f.s. FIG. 4.-Representation of crystal field and exchange splitting in the spin quartet state. Owing to a combination of (i) intrinsically different effective hyperfine interactions and (ii) markedly different relaxation rates specific Mossbauer patterns are expected depending on whether the I +3/2) or I & lf2) level is lowest lying i.e.whether D is negative or positive. For example under similar low-temperature circumstances in high spin Fe3+ 6S ions with an I S = 5/2 M = & 1/2) level lowest lying effective paramagnetic relaxation times and local perturbing fields preclude well-defined paramagnetic h . f . ~ . ~ In this fast relaxation limit one is left with a quadrupole doublet described by the nuclear spin Hamiltonian for the excited FeS7 level Completely analogous arguments apply to the present case of the I S = 3/2 Ms = + 1/2> doublet ; again a simple quadrupole splitting is expected when this is the ground doublet. No accurate description of the origin of the large EFG in these complexes is presently available.On the other hand when the I S = 3/2 Ms = +3/2) level lies lowest effective relaxation times z are long (zc% cu; l ) at low temperatures and a well-defined effective 26 SPIN 3/2 IRON COMPLEXES magnetic field is expected.s The resulting Mossbauer pattern is described by the nuclear spin Hamiltonian (for the excited state of FeS7 I = 3/2) the EFG tensor is expressed with respect to its principal axis system. The combined electric and magnetic interactions in this case lead to a multi-line pattern of normally six or more lines. Experimental data given below and appropriate to this case have been analyzed using a computer programme whose output are polycrystalline absorp- tion patterns for the Hamiltonian of eqn.(3). The general methods used to compute the spectra have been described 9 9 1 0 and in the present case all of the pertinent matrix elements have been given in closed form by Matthias Schneider and Steffen.' In the latter authors' work the spherical polar co-ordinates 4 and 8 are denoted a and p respectively. Except for this notational change the data of fig. 2 were analyzed with the conventions of ref. (11) and are summarized in table 1. The ground term in the S = 3/2 complexes is an orbital singlet so the hyperfine field arises from the core polarization interaction with S = 3/2. In the ground doublet 1 Ms = -&3/2) Hc is given by where (S,} = *3/2. Comparing eqn. (5) and (3) we find He, = -&*(a/glpN) ; it is easily shown that both " polarities '' yield the same Mossbauer pattern.By analogy with high spin Fe3+ 6S ions the core polarization term a is assumed negative. This description suffices to interpret the observed Miissbauer data in the para- magnetic complexes Fe(S2CNMe2)2CI Fe(S2CN(i-propyl),),Cl and Fe(S2CNEt,),Br. In the former two complexes the ME shows a many-line pattern a well-defined magnetic field (within the limit of relaxation broadening) and we conclude that the spin Hamiltonian parameter D is negative in these cases. This result is consistent with the e.p.r. data in the di-isopropyl derivative. The similarity of the spectra of (b) and (d) suggest that the zero-field splittings in these two complexes do not differ greatly. On the other hand the quadrupole doublet in the Fe(S,CNEt,),Br strongly suggests that D is positive ; the I S = 3/2 Ms = -& 1/2) level is lowest lying.The Mossbauer data while consistent with positive D in Fe(S,CNEt,)Br do not establish this result unambiguously. For example if the zero field splitting was extremely small I D Ilk< 1.2"K one would expect fast relaxation and only a quadrupole doublet independent of the sign of D. In the most general case then there are two possible explanations for the doublet of fig. 2(a); (i) very small I D I or (ii) comparatively large I D I and D> 0. In principle these two possibilities may be distinguished by a single crystal e.p.r. experiment at relatively large fields and low temperatures. In case (i) the small zero field splitting would not be expected to affect greatly the isotropic g = 2-0 resonance of the spin quartet level especially for large external fields.In case (ii) a resonance with gmaxN4*0 similar to that of Fe(S2CN(i-propyl),)Cl is expected. Unlike the latter case where the resonant doublet is an excited doublet the e.p.r. singal would be strongest at lowest tempera- ture for D> 0. E.p.r. experiments (24 Gc) at 4-2 and 1~4°K were therefore per- formed using a single crystal of Fe(S,CNEt,),Br. The magnetic symmetry axes H. H. WICKMAN AND A. M. TROZZOLO 27 of the crystal were not known prior to the experiment. The sample showed a single strong resonance whose intensity increased with decreasing temperature and which was characterized by g = 4.0+0.2 and gmin = 2.7+0.3. No resonance near g = 2 was found. This result argues for case (ii) a large (>2"K) and positive sign of D.The halo-constituent therefore appears crucial in determining the sign of the zero field splittings of the S = 3/2 manifold. The description of the h.f.s. of the Fe(S2CNEt2)2CI derivative is different as this complex undergoes a magnetic transition at 2-5"K3 The zero field splitting in this system has not yet been determined not is the mechanism of the collective ordering known in detail. Both dipole and exchange effects undoubtedly contribute to the ordering; we refer to these interactions collectively as the exchange field Hexch. Depending on the relative strengths of the crystal field and exchange field conplicated behaviour may occur. As a start however we will assume that the zero field splitting in Fe(S2CNEt,)2C1 is similar to that in the closely related Fe(S,CN(i-propyl),),C1 derivative.Because the ordering occurs at 2*5"K the magnetic character of the ion is mainly determined by the ground doublet i.e. the I Ms = +3/2) level. Here the relations I (S,) I = 3/2 (S,} = (S,,) = 0 are valid. The exchange inter- action may then be represented in the Weiss molecular field approximation by a field Hexch directed along the z-axis of the spin Hamiltonian of eqn. (1). Under these conditions a relatively simple level scheme is found. As shown in fig. 4 the exchange splitting simply removes the degeneracy of the I +3/2) level. The transition temperature of 2.5"K is a reasonable estimate of the separation of the two levels at the experimentai conditions of 1-2"K; the upper state populations are sufficiently low to be neglected entirely. When relaxation times are long (2 % C O ~ l) the I Ms = +3/2) and 1 Ms = -3/2) levels separately contribute identical Moss- bauer spectra and the observed spectrum is a Boltzman sum of these two spectra.The Mossbauer pattern of a polycrystalline absorber is (neglecting polarization of the y-rays) indistinguishable from the magnetic h.f.s. from the unsplit I M = &3/2> doublet. (If polarization of the y-rays were observed the two could often be dis- tinguished i.e. the transitions in a paramagnetic ion with {S,) = 0 are unpolarized.) The great similarity of the Mossbauer h.f.s. in Fe(S2CN(i-propyl)2)2Cl and Fe(S2CNEt2),C1 offers substantial support for a simple exchange splitting of the ground doublet. Deviations from the model would in principle be shown primarily by differences between the observed core polarization fields He,,.The latter para- meter is proportional to (S,) in one of the lower two electronic levels of the ion; an exchange field at an angle to the z-axis of the electronic spin Hainiltonian would produce a mixing of the I Ms = 1 3/2) and I M = k 1/2) levels and thus change (S,) from the value found in paramagnetic case. In fact no experimental evidence evidence for such an effect was observed the fields He, in both the paramagnetic and ferromagnetic samples were essentially the same. The large quadrupole splittings in these compounds are not as easily interpreted as the magnetic interactions. Within two Kramers doublets arising from an orbital singlet and spin quartet level there is no temperature dependence to the EFG i.e. the net EFG at the nucleus does not change with the population of the two Kramers doublets.Because of this no direct determination of the ionic contribution to the EFG from the quartet term is possible. The relatively temperature independent quadrupole splitting is consistent with the absence of excited electronic terms within a few hundred cm-I of the ground state. We have noted previously that the levels of the lowest quartet term in octahedral symmetry 4T1 separately produce no EFG at the n u c l e ~ s . ~ The lattice contribution to the EFG may be large enough to account for the large splittings but it seems more likely that an appreciable gradient 28 SPIN 3/2 IRON COMPLEXES should arise from the ground quartet term. In this case the orbital level will be a mixture of states from different octahedral representations.A larger quadrupole splitting was observed in the bromo derivative than in the chloro-complexes; this effect could equally well be accounted for by a change in either the ionic or lattice contribution to the total gradient. In all of the chloro derivatives the EFG q was positive with the assumption that eQ is also positive. In earlier work6 it was shown that at higher temperatues where the main effect of relaxation is to broaden the quadrupole peaks the left peak is broadened more than the right peak. This results mainly from the fact that within the ground Kramers level the effective field fluctuates along the axis perpen- dicular to the principal axis of the EFG.12 For collinear EFG and effective internal field the right peak would be broadened. This may be seen qualitatively as follows.With the nuclear quantization axis along the 2'-direction (eqn. 3) the excited state nuclear wave functions are (with 7 = 0) I 1/2) and I & 3/2) and correspond with q> 0 to the left and right hand peaks in a pure quadrupole pattern when (Heff} = 0. These two levels may be described by eflectiue nuclear g factors with I' = 1/2 in each case. Their magnetic character is described by the respective g' tensors (9 = g; = 2gl ; gl = gl) and (9 = gi = 0 ; gi = 3g,). Hence the I & 1/2) level will respond more readily to transverse fluctuations of a magnetic field (the present case) while the I +3/2) is affected first when the effective field fluctuates along an axis parallel to the EFG principal axis. These considerations emphasize the result that it is not in general possible to deduce the absolute sign of an EFG from relaxation broadening of a quadrupole doublet without knowledge of the relative orientation of EFG principal axis and the magnetic hyperfine direction.Finally while the qualitative picture involving effective g values is useful in that it predicts correct features of the spectrum it is rigorously incorrect in that it assumes a static perturbing field while in fact the fields are of a stochastic nature and models based on the ideas are generally required to give definitive answers relating to origins of broadening in Mossbauer spectra. In the bromo derivative where the I & 1/2) level is lowest and relaxation times very fast the effect of an external magnetic field is to induce an ionic moment in the paramagnetic ion and hence an internal field owing to the core polarization mechanism.For simplicity we assume that only the 1 f 1/2) ground doublet is occupied. Because little other information is available a main justification will lie in the agreement of this analysis with the experimental results. The ground doublet has 911 = 2 gl = 4 and (9) = g' = 2J3. An external field Hext will induce an internal magnetic field given by where 3 P H e x t (9;;") 2 k T ' (Sz}ion = - tanh - - - - J3 2 for P H I k T d . In the present case of Hext = 7 kOe T = 3.0°K it follows that Hint E -42 kOe. Because Hi, is oppositely directed to Ifext the net hyperfine field is -35 kOe. Calculations of the broadening of quadrupole lines induced by a random magnetic field have been made by Gabriel and Ruby,9 and C01lins.l~ Comparison of their computed spectra allow us to estimate 1 He, 1 from fig.3 to be 34+5 kOe. This result is in good agremeent with the estimate made using the single Kramers level H. H. WICKMAN AND A. M. TROZZOLO 29 approximation. In the present experiment the large internal field was produced by a relatively small external field. This method differs from the situation for which the calculations of Gabriel and Ruby and Collins l3 were intended namely where the large magnetic field was produced externally by a superconducting magnet. Finally the agreement of the approximation used to interpret the present data sug- gests that the splitting between the two Kramers doublets is greater than 6°K. B. F. Hoskins R. L. Martin and A. H. White Nature 1966 211 627. R. L. Martin and A. H. White Inorg. Chem. 1967 6 712. H. H. Wickman A. M. Trozzolo H. J. Williams G. W. Hull and F. R. Merritt Physic. Rev. 1967 155 563. H. H. Wickman and F. R. Merritt Chem. Physics Letters 1967 I 117. H. H. Wickman and A. M. Trozzolo Znorg. Chem. in press. 1966 16 162. ' H. H. Wickman and A. M. Trozzolo Physic. Rev. Letters 1965 15 156 ; Physic. Rev. Letters ' G. K. Wertheim and J. P. Remeika Physics Letters 1964 10 14. * H. H. Wickman M. P. KIein and D. A. Shirley Physic. Rev. 1966 152 345. J. R. Gabriel and S. L. Ruby Nucl. Instr. Methods 1965 36 23. lo H. H. Wickman and G. K. Wertheim Physic. Rev. 1966,148 211. l1 E. Matthias W. Schneider and R. M. Steffen Arkiv Fysik 1963 24,97. M. Blume Physic. Rev. Letters 1965 14 96. l3 R. Collins J. Chem. Physics 1965 42 1072,

ISSN:0430-0696    

DOI:10.1039/SF9670100021

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

H. H. WICKMAN AND A. M. TROZZOLO 29 GENERAL DISCUSSION Prof. N. N. Greenwood (Newcastle upon Tyne) said Two of the most fascinating results in Wickman’s paper are (i) that a change from chlorine to bromine in these compounds alters the sign of D the splitting of the lowest quartet level ; and (ii) that in the chloro-derivative the diethyl compound is ferromagnetic whereas the dimethyl and di-isopropyl compounds are paramagnetic at low temperatures. Is there any explanation of these observations and in particular is it possible to give them any intuitive chemical rationalization ? Dr. H. M. Wickman (Bell Telephone Lab. N.J.) said. In reply to Greenwood a possible explanation of the change in sign of D has been suggested by preliminary results of a crystal field calculation for C, symmetry within the 3d5 configuration representing penta-co-ordinate Fe (111).There are many limitations in this approach and the calculation though a straightforward application of results of Tanabe Sugano and Kamimura is tedious. However for a reasonable choice of parameters specifying the ligand field potential Racah parameters etc. I find that a cross-over occurs between two ground quartet terms as the parameter representing halide charge is varied in a systematic fashion. As spin-orbit interaction was neglected in the calculation the actual splitting within these quartet terms has not been directly computed. On the basis of the cross over however it appears likely that the change in sign of D in the S = 3 spin Hamiltonian in the ethyl-chloro and ethyl-bromo derivatives arises from the appearance of a different ground quartet in the two cases.This situation may be contrasted with the high spin Fe3+ 6S where inversion in sign of D must always be due to variable contributions to the zero-field splitting with the (unique) S = 3 ground term. It should be emphasized that it is not yet possible completely to rule out this possibility in the quartet case. To do this explicitly would require additional data such as optical studies. 30 GENERAL DISCUSSION There are two points which should be noted concerning the paramagnetism of most intermediate spin complexes with respect to the ferromagnetism of the ethyl- chloro derivative. First the molecules containing one iron ion each are apparently packed in the crystal at the Van der Waals radii so that direct or super-exchange in the usual sense must be very weak.Secondly the smallest iron-iron separation is slightly less than 7A and as a result dipolar fields are also weak. Because of the easily polarized sulphur atoms and the associated 71 system of the molecule it is conceivable and perhaps in this case probable that the interaction leading to ferro- magnetism should be termed intermolecular exchange. If true and further experi- ments and other investigations are under way to attempt to clarify this point the lack of ordering in most of the complexes would merely be an expression of the sensitivity to orientation and molecular overlap of this intermolecular exchange mechanism. One must note that the above argument is predicated on the assumption that no crystal phase change or other macroscopic change occurs in the ethyl-chloro derivatives between room and helium temperatures.The magnetic and Mossbauer data do not indicate that any such change is occurring. Dr. C. E. Johnson (A.E.R.E. Harwell) said Would Wickman like to say more about his crystal field calculations and especially how he decides what charges to put on the ligand? Dr. H. H. Wickman (Bell Telephone Lab. N.J.) said As mentioned previously these calculations are preliminary and have been carried out only for one case thus far. For the results quoted above the " charge " on the sulphur was taken such that if there were six sulphurs octahedrally coordinated about the iron the cubic field strength would be about 85% of that required for a 6A1 to 2T2 cross-over. (This initial choice was suggested by experimental data in tris-dithiocarbamates where there is in fact such a cross-over in octahedral coordination of six sulphur ligands.) The halide charge was then varied for a range of strengths encompassing the fixed charge of the sulphur.

ISSN:0430-0696    

DOI:10.1039/SF9670100029

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

Magnetic and Electric Hyperfine Interactions of FeS7” Nuclei in Compounds Y3- Ca Fe,- Sn O, (O.O<x<2-0) BY I. S. LYUBUTIN* E. F. MAKAROV~’ and V. A. POVITSKII~’ Inst. of Chem. Physics and Crystallography Institute Academy of Science Leninskii prospekt Moscow U.S.S.R. Received 27th July 1967 The effective magnetic fields at the Fe5’ nuclei in the octahedral and tetrahedral sub-lattices of the tin-substituted YIG YJ~xCaxFe5~xSn,012 (0.0 < x< 2.0) were investigated by the Mossbauer method. The dependence of Heff on x obtained from the experimental results is compared with the correspondent dependence of the magnetic moments of the Fe3+ ions derived from the Gilleo theo- retical model. The reasons for the difference in the concentration dependence of Heff and the magnetic moment for the a-sub-lattice are discussed.The contribution to the Heff from the dipolar interaction of neighbouring magnetic ions was negligible in the iron garnets. The main reason for the difference in the values of Heff in the a- and d-sub-lattices of the iron garnets is connected with the fact that the chemical bonding of Fe3+ ions in tetrahedral sub-lattice is partly covalent. The quadrupole effects in YIG could not be observed on the polycrystalline samples below the Curie temperature. The quadrupole splitting and the isomer shifts for the both sites of Fe3+ at T>T were observed. Froin the line intensity the distribu- tion of Sn4+ and Fe3+ ions in the sub-lattices was obtained. It is known that Fe3+ ions are located on the two crystallographically non- equivalent sites in different quantities in the yttrium-iron garnet (YIG) structure.Three Fe3+ ions are at the tetrahedral sites (d-sub-lattice) and two Fe3+ ions are located on octahedral sites (a-sub-lattice) in the formula unit (Y3][Fe,] (Fe,) O12. In the YIG substituted by tin it appeared possible to measure the Mossbauer effect on the nuclei of magnetic (Fe57) and non-magnetic (Sn119) ions and to study the effective magnetic fields He, acting on those nuclei in the different sub-lattices. The special interest of this system is due to the presence of the large effective magnetic fields at the Sn119 nuclei.1-4 This paper deals with the investigation of the Mossbauer effect on the FeS7 nuclei in polycrystalline samples of the Sn4+-substituted yttrium-iron garnets Y,-xCa,Fe,-,Sn,O, (x == 0 ; 0,l; 0,3; 0,5; 0,7; 0,9; 1,O; 1,l ; 1,2; 1,5 and 2,O).The source CoS7 in a Cr matrix was at the room temperature. MAGNETIC HYPERFINE INTERACTIONS In fig. 1 the Mossbauer absorption spectra of Y3-,Ca,Fe,-,Sn,01 for different values of x at 77°K are shown. The spectra up to x = 1.1 can be interpreted as two Zeeman effects arising from the Fe57 nuclei in the a- and d-sub-lattices where they are subjected to the different effective magnetic fields. We consider that the more intensive lines are produced by the d-~ub-lattice,~ where there are more Fe3+ ions than in the a-sub-lattice. When x = 1.1 on the *Crystallography Institute Academy of Sciences U.S.S.R. Moscow. ?Institute of Chemical Physics Academy of Sciences U.S.S.R. Moscow. 31 32 MAGNETIC AND ELECTRIC HYPERFINE INTERACTIONS Zeeman background an intensive doublet appears; this is due to the transition of some part of the Fe3+ ions to a paramagnetic state.When the magnetic splitting is close to zero the spectra are transformed to give the asymmetric doublet. Non-magnetic ions Sn4+ are located in the octahedral a-sub-lattice in the Y3-,Ca,Fe,-,Sn,01 system.6 The part of Fe3+ ions in the tetrahedral d-sub-lattice have as first neighbours (in the a-sub-lattice) only nonmagnetic ions Sn4+ when x= 1.2 v mm/sec FIG. 1 .-Mossbauer absorption spectra of the iron garnets Y3-xCaxFe5-,SnxO12 for different values of x at 77°K. Sn4+ ions are randomly distributed at the a-sites. These Fe3+ ions are in the paramagnetic state because d-a-interaction is the main effect in the iron garnets. We consider that the appearance of the doublet in the central part of the Moss- bauer spectrum is connected with the Fe3+ ions in the d-sub-lattice ; these are excluded from exchange interaction since their first neighbours are non-magnetic Sn4+ ions.The dependence of the effective magnetic field at the nuclei Fe3+ ions in a- and d-sub-lattices on x at 77°K is shown in fig. 2. In the interval O<x< 1.1 the field behaviour for both sub-lattices is similar. Then values of the field sharply decreases over the small interval of x 1.1 <x< 1.2 and only the broadening of the doublet lines shows that in the compounds Y1.,Cal.2Fe3.8Snl.2012 (T = 290°K) and Y1.5Ca1.5Fe3.5Sn1.5012 (T = 140°K) ' the effective magnetic field is still acting at the Fe nuclei. This field disappears only when x = 2.0. The existence of this field up to x = 2-0 may be explained by the appearance of the superparamagnetic regions for 1-2 < x < 2.0 in the ferrite.The superparamagnetic transition at 130°K for the compound Y1.5Cal.5Fe3.5Sn1.5012 was predicted recently by Ishikawa.' 1. S. LYUBUTIN E . F . MAKAROV AND V. A . POVITSKII 33 It is interesting to compare the values of the effective magnetic fields with the values of the magnetic moments of the sub-lattices. To estimate these magnetic moments we used the theoretical model of Gi1le.0~ Fig. 3 shows the theoretical curves (lines) of the magnetic moments corresponding to one Fe3+ ion in the a- 0 0 . 5 1.0 I *5 2.0 X FIG. 2.-The dependence of the effective magnetic field Hcff at the nuclei of the Fe3f ions in a- and d-sub-lattices of Y3-xCaxFe,-,SnxOlz against x at 77°K.Also shown is the concentration dependence of the Htff at the Sn1I9 nuclei in these garnets3 X FIG. 3.-The theoretical dependence of the magnetic moments of the Fe3+ ion in the a- and d-sub- lattices (lines) and values of magnetic moments obtained from the experimental values of the Heff and d-sub-lattices calculated by us using Gilleo model for the (Y,-xCax)[Fe,-,Snx] (Fe,) OI2 garnets. The value of Heff depends on the effective magnetic moment of the ion.'' Assuming that the effective magnetic field H& in the d-sub-lattice is proportional to the magnetic moment of Fe3+ ion in this sub-lattice we have calculated the values of the magnetic moments of d-sub-lattice Md(x) for different x using the measured (crosses) against x for the Y3-xCaxFe5-xSnxOl system. 2 34 MAGNETIC AND ELECTRIC HYPERFINE INTERACTIONS values of H&(x).The values of the magnetic moments of Fe3+ ions in the d-sub- lattice obtained from the relation are shown in fig. 3 (crosses); where Md(0) = 5pB-is the magnetic moment of Fe3+ ion and H&(O) is the field at the nucleus of this ion in the d-sub-lattice of the pure YIG (x = 0). The values of the calculated magnetic moments are in a good agree- ment with the theoretical values in the interval O<x< 1.0 (fig. 3). The theoretical values of the magnetic moment of Fe3+ ion in the a-sub-lattice do not depend on the substitution and are equal to 5,uB for all x (fig. 3). However from experimental data He, (at 77°K) in the a-sub-lattice decreases when x rises in the same way as in the d-sub-lattice (fig. 2). Analysis shows that this result cannot be explained using only the temperature effects i.e.by the approach of the Curie point to the temperature of the measurement. The decrease of He, in the a-sub-lattice in comparison with He, expected theo- retically from values of the magnetic moments of Fe3+ can be probably explained by the " reverse " influence of the d-sub-lattice on the a-sub-lattice when the substitution takes place in the a-sub-lattice. The Gilleo theory does not take into consideration this influence. Actually according to the Gilleo model when the magnetic ions are substituted by non-magnetic ions in the a-sub-lattice the effective magnetic moment of Fe3+ ion in the d-sub-lattice decreases with decreasing number of the exchange linkages with the magnetic ions in the a-sub-lattice.However the variation in the value of the effective magnetic moment of the magnetic ion may probably take place not only due to the decrease of the number of the exchange linkages but also due to the decrease of the effective magnetic moment of the neighbouring ion with which the exchange interaction takes place. We now discuss the differences between He, at the nuclei of Fe3+ ions in octahedral site and Heff in the tetrahedral site of YIG. Freeman and Watson lo suggested that possible reasons for the decrease of He, at the nuclei of the Fe3+ ions in the d-sub-lattice in comparison with the field in the a-sub-lattice were (i) distortions of cubic symmetry (ii) covalent bonding effects. For the pure YIG this difference is aboutc90 kOe and when we change the concentration of the Sn4+ ions this difference remains approximately constant up to x = 1.0 (fig.2). However at the same time when x rises the number of the magnetic ions in the a-sites decreases (because they are substituted by Sn ions) which are the nearest neighbours for the d-sites. This should decrease the value of the dipolar field in the d-sub-lattice and consequently decrease the difference between He, in the a- and d-sites. As the measured difference in the He, in the a- and d-sites remains constant up to x = 1.0 (see fig. 2) in spite of the fact that the number of the neighbouring magnetic ions which cause the dipolar field decrease two-fold for x = 1.0 we may conclude that the dipolar fields slightly influence the He, value. The values of the isomer shifts for Fe3 ions in the tetrahedral and the octahedral sub-lattices of the garnets Y,-,Ca,Fe,-,Sn,O, and unusual large value of the quad- lupole constant (see table 1) confirm that the chemical bonding with oxygen ions has partly the covalent character l1 of Fe3+ in the tetrahedral sites.This covalent bonding is probably the main reason of the difference between He, in the octahedral and tetrahedral sub-lattices of rare-earth iron garnets. When the x rises the values of the isomer shifts and the quadrupole constants for the a- and d-sub-lattices of the Sn4+-substituted YIG remain approximately constant (see later). This result explains the constancy of the difference between He, in the a- and d-sub-lattices up to x = 1.0 1. S . LYUBUTIN E . F . MAKAROV A N D V . A . POVITSKII 35 ELECTRIC HYPERFINE INTERACTIONS Alf and Wertheim l2 were the first to measure the quadrupole interaction of Fe5' nuclei in a single crystal of the YIG using the Mossbauer method in an external magnetic field of 0.5 kOe applied along the [ 11 11 and [ 1001 axes.The measurements were made at 300'K i.e. lower than the Curie point (Tc = 550°K). No quadrupole interaction was found in the polycrystalline samples of the YIG at 300°K nor at 80'K,5 and thus the authors concluded that e2qQ = 0. Our measurements at the temperatures T<Tc also do not reveal any quadrupole interaction. We explain this contradiction in the following way when electric quadrupole and mag- netic dipole interactions exist simultaneously and e2qQQp.H and I = 3/2 the (e'4Q) megwed e2qQ (3 cos*O- 1)/2 where 8 is the angle between He, direction (which coincides with the direction of the easy magnetization axis in the absence of the external magnetic field) and the direction of the symmetry axis of the electric field gradient (EFG).13 When 8 = 54'44' the term 3 cos20- 1 = 0 and consequently (e2qQ)measured = 0- This case is realized in polycrystalline samples of YIG at temperatures below 7'.In fact in the YIG the easy magnetization axis is in the [ l l 11 direction l4 while the axis of symmetry of the EFG for the d-sub-lattice coincides with the [loo] direction and one for the a-sub-lattice coincides with the [ I l l ] direction. Thus in the absence of the external magnetic field the angle 8 is 54'44' for all Fe3+ ions in the d-sub-lattice and in the a-sub-lattice 6 = 70'32' for 75 % of the Fe3+ ions i.e. (e2qQ)measured = 4 e2qQ and 6 = O only for 25 % of Fe3+ ions i.e.Thus observation of the quadrupole interaction at the Fe57 nuclei in poly- crystalline YIG below Tc is practically impossible. The precise values of e2qQ for both sites of Fe3+ -ions in this case can be obtained in the paramagnetic state. Such measurements were made on some rare-earth iron garnet.15 Mossbauer spectra of the different tin-substituted YIG at temperatures T > T are shown in fig. 4. All of them except the last consist of 3 lines of different intensity. The width of outside lines of the spectra are always the same and in the pure YIG they are equal to 0.3 rninlsec and in the substituted garnets 0.4+-0-45 mm/sec. These spectra may be interpreted as consisting of the two quadrupole doublets where the lines corresponding to the larger velocities coincide and so form the most intensive third line of the spectrum.The more intensive doublet with the greater splitting and the lower isomer shift belongs to the Fe3+-ions in the d-sub-lattice. The results of the present work on pure YIG are shown in the table 1 with the data from ref. (ll) (12) (15). When the tin content rises T decreases and for x = 1.0 T = 2 6 0 K 6 Thus for this sample we can measure the temperature shift in the 300-573°K range. The value of the shift is 0.08+0.03 mm/sec for both Fe3+ sites. On the other hand the isomer shifts for both sites of Fe3+ are almost indepen- dent of the tin content. This allowed one to correct the data of Nicholson and Burns (see table 1) in which the isomer shifts of YIG have been measured at 610"K but corrected for 300°K using the value 0.22 mnilsec obtained for pure iron.I6 For the spectra of the tin-substituted YIG the decrease of the intensity of the second and the third lines of the spectrum is observed with x increasing (see fig.4). When x = 2 the second line almost disappears. We therefore suggest that the remaining two lines correspond to Fe3+ ions in the d-sub-lattice only. Their areas are equal. The doublet of the a-sub-lattice appears to be asymmetrical with a ratio of the areas of 1 0.75. This asymmetry is probably connected with the anisotropy of the Debye-Waller factor for Fe3+ ions in the a-sub-1atti~e.l~ The comparison of (e2qQ>measured e'qQ. 36 MAGNETIC AND ELECTRIC HYPERFINE INTERACTIONS the total areas of the components of the both doublets in the pure YIG (when the relative number of the Fe3+ ions in the both sub-lattices is taken into account) allows an estimate of the ratio of the Mossbauer probability in the sub-lattices fa/h=l.l.This ratio probably does not change when substitution takes place. The total number of Fe3+ ions corresponding to the formula unit Y,-,Ca,Fe,-,Sn,012 3 I ~ s " ~ ~ k ~ .~ .r 7 '"-"**-*-. :.,. ~". ~ ~ ~ ',,. ,.'.*.."*..'. I 1 3 5 .-*.':;.. . p #. * . f 33 # . . . *...-. . . . 5 . '.-. XE2.O 29 X=0.9>-. * *.' I 1 -1.0-0.5 0 0.5 1.0 -1.0-0-5 0 0.5 1.0 velocity (mrnlsec) FIG. 4.-Mossbauer spectra for two Fe3f ions sites of Y3-xCaxFes-xSnxO12 in the paramagnetic temperature region. TABLE 1. quadrupole A = 1.e2qQ splitting (mmlsec) isomer shifts d* at 300°K (mmlsec) a-site d-site a-site d-site data l2 0.94 f0.19 0.78 f0-16 0.57 f0-0511 0*26f0.0511 data l5 0.52 f0-04 0.92 f0-04 0.49 f0.04 0.32 f0-04 present work 0.47 f0.02 0-93 f0-02 0.46 f0.05 0.23 f0.05 T = 300°K - - T = 610°K 0-36 f0.07t 0.19 f0.071- T = 573°K *relative to stainless steel ; t the values corrected by us (see text).changes from 5 when x = 0 to 3 when x = 2. Hence as above we can obtain the number of Fe3+ ions in each sub-lattice from the experimental spectra (see fig. 5). These results prove that Sn4+ ions substitute for Fe3+ ions in the octahedral sub-lattice only. This fact confirms the data of Geller et aL6 obtained earlier by X-ray investigation for x = 1 and x == 2. The Mossbauer method is however simpler and more convenient than X-ray analysis for studying substitution in rare- earth iron garnets. I .S. LYUBUTIN E . F . MAKAROV AND V. A . POVITSKII 37 The isomer shifts and the quadrupole interactions for both sites of Fe3+ ions were almost independent on the tin contents in garnets. The quadrupole splitting for both a- and d-sub-lattices is independent on temperature in the range from the Curie point to 600°K. X FIG. 5.-Number of Fe3+ ions in the a- and d-sub-lattices of Y3-,CaxFe,-xSnxO12 against the concentration of the tin ions x. We thank Prof. K. P. Belov Prof. L. M. Belyaev and Prof. V. I. Goldanskii for their interest in this work and for useful discussions. We are grateful to Y. V. Baldochin for help in performing the experiment. K. P. Belov and I. S. Lyubutin JETP Letters 1965 1,26. V. I. Goldanskii M. N. Devisheva V. A. Trukhtanov and V. F. Belov JETP Letters 1965,1 31 ; Physics Letters 1965 15 317.K. P. Belov and I. S. Lyubutin JETP 1965,49 747. V. I. Goldanskii M. N. Devisheva E. F. Makarov G. V. Novikov and V. A. Trukhtanov JETP Letters 1965 4 63. R. Bauminger S. G. Cohen and A. Marinov S. Ofer Physic. Rev. 1961 122 743. S. Geller R. M. Bozorth M. A. Gilleo and C. E. Miller J. Physics Chem. Solids 1960,12,111. K. P. Belov and I. S. Lyubutin Kristullographiya 1965 10 351. M. Gilleo J. Physics Chem. Solids 1960 13 33. * Y. Ishikawa J. Appl. Physics 1964 35 1054. l o R. E. Watson and A. J. Freeman Physic. Rev. 1961 123 2027. I1 L. R. Wakler G. K. Wertheim and V. Jaccarino Physic. Rev. Letters 1961 6,98. l2 C. Alf and G. K. Wertheim Physic. Rev. 1961 122 1414. l3 G. K. Wertheim M&buuer Eflect (Acad. Press. N.Y. London 1964). l4 J. F. Dillon Physic. Rev. 1958 111 1476. l6 R. S. Preston S. S. Hanna and J. Heberle Physic. Rev. 1962 128 2207. I7S. V. Karaygin Doklady 1963,148 1102. W. J. Nicholson and G. Burns Physic. Rev. A 1964,133.

ISSN:0430-0696    

DOI:10.1039/SF9670100031

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

GENERAL DISCUSSION Prof. J. J. van Loef and Dr. A. M. van der Kraan (Reactor Institute Delft) said It is suggested in the paper by Lyubutin et al. that the observation of the quadrupole interaction at the 57Fe nuclei in polycrystalline YIG below T is practically impossible. In analyzing our Mossbauer spectra of polycrystalline YIG at room temperature shown in fig. 1 we come to the following conclusions.1 The measured magnetic FIG. 1 .-The Mossbauer spectrum of polycrystalline YIG as an absorber and 57C0 in Pd as a source. The arrows indicate the line positions of the various hyperfine components. hyperfine spectrum of YIG comprises three different Zeeman splittings two for Fe3+ ions in the a-sublattice and one for Fe3+ ions in the d-sublattice. The two a-site hyperfine spectra are characterized by two quadrupole interactions of +e2qQ = + (0.05&0*015) mm/sec for 75 % of the Fe3+ ions and -(0-12f0.03) mm/sec for 25 % of the Fe3+ ions respectively.Furthermore the hyperfine fields at these two groups of Fe3+ ions are different by (6.2+ 1*7)kOe which is in the first place due to the inequivalence of the octahedral iron sites because of dipolar fields. This is in good agreement with the results of n.m.r. experiments by Meyer and co-workers,2 who found a difference of (6.6+0.3)kOe. The d-site hyperfine spectrum is a single six-line Zeeman splitting with no quadrupole interaction in line with the results reported by Lyubutin. We have compared the Mossbauer spectra obtained with polycrystalline YIG with that found in a single crystal without applying a magnetic field.Both the single crystal and the powdered sample give the same results. Therefore it is con- cluded that even in polycrystalline samples of the garnet the electric hyperfine inter- action measured at temperatures T<T can be fully understood. In fact from the correlation of the quadrupole coupling constant with the magnetization vector the easy direction of magnetization follows in a straightforward manner. We have applied this method to determine the easy direction in Sm1G.l The major reason why our YIG results deviate from those reported by Lyubutin and co-authors is probably because our counting rates per velocity channel of about lo6 are by an order of magnitude higher than theirs. J. J. van Loef Proc. Int. Con$ Magnetism (Boston 1967). R. Gonano E. Hunt H. Meyer and A. B. Harris J.Appl. Physics 1966 37 1322 and private communication. 38 GENERAL DISCUSSION 39 Prof. N. N. Greenwood (Newcastle upon Tyne) said Is it possible to apply Gilleo's theory to measurements taken only at 77"K? Lyubutin states that dependence of the Curie temperature on x cannot explain the results by itself but surely this dependence must play at least some part in the analysis. It also seems strange that the isomer shifts and quadrupole interactions for both sites of Fe3+ ions are almost independent of the tin content of the garnets. Is there any explanation of this? In particular the electric field gradient would be expected to be sensitively dependent on lattice distortions brought about by the varying tin content. Dr. R. Krishnan (Laboratoire de Magnitisme France) said With regard to Lyubutin et aZ.'s paper when one interprets the effect of substitution of non-magnetic ions on the hyperfine fields of garnet and spinel ferrites in terms of the variation in the number of magnetic neighbours of Fe3+ in the two sites one is faced with cer- tain difficulties.Fur example in the present case of Y3-,Ca Fe,-,Sn,0,2 the hyperfine fields at 77°K for both a and d sites decrease as x increases whereas in many cases such as Nil-,Zn,Fe204 studied by Goldanski et aZ. and Nil+ GeXFe2-2,O4 studied by Fatseas and Krishnan the hyperfine field at only one site varies. In the latter system we have shown that the hyperfine field at A sites decreases as x increases whereas that at B sites remains constant. We had attributed this to the increased degree of covalency in Fe3+-02- bonds for A sites by virtue of the presence of Ge4+ ions in these sites. Thus we see that the nature of chemical bonds is more important than just the number of magnetic linkages (Gillea model). T. Fatseas and R. Krishnan Int. Con$ Magnetism Boston Sept. 1967.

ISSN:0430-0696    

DOI:10.1039/SF9670100038

出版商:RSC    

年代:1967

数据来源: RSC

摘要:

Mossbauer Effect in the System Srl-,La,Fe03 BY P. K. GALLAGHER AND J. B. MACCHESNEY Bell Telephone Laboratories Incorporated Murray Hill New Jersey Received 1 1 th September 1967 Massbauer spectra at room temperature of samples in the system Srl-,Lax FeOBshow a single absorption peak for x = 0.2-0-6 having an isomer shift intermediate between that of iron@I) and iron(N). This is the result of an electronic exchange which is rapid compared to the lifetime of the excited state of 57Fe. Consequently the Mossbauer effect is indicative of an average oxidation state. At 78"K however Mossbauer spectra of these samples show magnetic hyperfine splitting and the individual oxidation states are readily resolved. The temperature dependence of the Mossbauer effect is indicative of the onset of the rapid electronic transfer and it is discussed with respect to the different phases within the system.In earlier studies of the Mossbauer effect of materials containing iron(IV) the authors have investigated systems in which iron(II1) and (IV) coexist in various crystalline phases. These phases are ideally of the type A~+Fe~40,+2,. Charge compensation for iron (111) present in these systems is achieved by the creation of oxygen va~ancies.''~ It is therefore of interest to compare behaviour in these systems with analogous ones in which compensation by anion vacancies can be eliminated. The system Sr,-,La,FeO was chosen for such a study as it is ideally suited for such an investigation especially since the end number of this series SrFeO, has already been well characterized.l* During the course of this work Shimony and Knudsen described the Mossbauer effect in this system and called attention to the appearance of a rapid electronic exchange between the iron ions.Such an exchange gives rise to an average oxidation state 3- of the same type as that observed for iron(I1) and iron(II1) ions on the octahedral sites in magnetite.6s8 This effect can arise if the electronic exchange is rapid compared to the time of measurement for an individual event in the Mossbauer effect. Shimony and Knudsen were unable to resolve the oxidation states even though they performed some measurements at 78°K. In this work however resolution was observed at this temperature in the initial cursory inspection of the Mossbauer effect in this system. The most obvious examples of samples showing an " average " oxidation state are those where the amounts of iron(II1) and (IV) are nearly equal (0.4 < x < 0.6).These samples are antiferromagnetic with Nee1 temperatures of 190-200°K. Consequently they exhibit magnetic hyperfine splitting at 78°K and it is the primary purpose of this work to investigate the temperature dependence of the Mossbauer effect in greater detail and thereby determine if this resolution of oxidation states is associated with the magnetic ordering process or whether it is simply the result of the temperature dependence of the electronic transfer process. Ceramic samples of Sr,-,La,FeO were prepared with x = 0.2 0-4 0.5 0.6 0.8 and 1.0. Appropriate mixtures of Fe,03 SrCO and La203 were calcined at 1200°C ground and pressed into discs to be fired at 1240°C for final sintering.However the high temperatures needed to prepare these specimens cause oxygen 40 P . K . GALLAGHER AND J . B . MACCHESNEY 41 deficiency especially in strontium-rich specimens. This was restored by anneal- ing at moderate oxygen pressures (lo3 atm) and 618°C for one week. X-ray diffraction patterns of these specimens were obtained with CrKa radiation using straumanis-type Norelco cameras (1 14-59 mm diam.). Cell constants were calculated by the least-squares refinement of Mueller Heaton and Miller. Analyses for available oxygen yielding the ratio of trivalent to tetravalent iron was performed by the method described previ~usly.~ These results are summarized in table 1. TABLE 1 .-COMPOSITION AND CELL CONSTANTS FOR SPECIMENS Srl,La,FeO3 X % Fe4+ symmetry a0 b0 CO Y .O* 100 cubic 3-850 *2 86 cubic 3.865 -4 67 cubic 3.880 *5 48 rhomb 3.889 90-26 -6 45 rhomb.3.896 90.333 -8 34 ortho? 5.510 5-640 7-81 5 1 *ot - ortho. 5.556 5.565 7.862 *ref. (1); tS. Geller and E. A. Wood Acta Cryst. 1956 9 563. Mossbauer spectra of these materials were measured on an apparatus similar to that described by Wertheim.' Temperatures were controlled by using an Andonian Dewar and control system. Temperatures were measured by means of a calibrated platinum resistance thermometer and maintained within & 0.1 "K. The furnace used for above ambient measurements is described elsewhere.' Calibrations were based upon the ground-state splitting of 57Fe and a value of 3-92mmsec-' was used from n.m.r. measurements.'2* l3 Isomer shifts of magnetically split spectra were determined from the centroids.The source was 57C0 in palladium and all values of isomer shift reported are relative to this material. The samples were prepared by making a paraffin slurry of an unweighed amount of material and spreading it on 0.005in. thick aluminum foil. An absorption due to iron impurities in the aluminium foil and beryllium windows was substracted from the spectra. We are concerned principally with the electronic exchange aspects of the Sr,-,La,FeO system therefore samples having values of x = 0.4-0.6 will be stressed and only a brief survey of the results for the samples in which x = 0.2 0-8 and 1.0 will be presented. Fig. 1 shows data used to determine accurately the N6el tempera- ture of LaFeO,. The counting rate was measured at zero velocity for a variety of temperatures.The sudden decrease in counting rate corresponds to the collapse of the magnetic hyperfine structure above the Nee1 temperature. This temperature is confirmed by breaks in the differential thermal analysis trace (10°C min-l) made in this region as shown in the insert of fig. 1. The Nee1 temperature is clearly near 750°K. A summary of the magnetic hyperfine splitting and isomer shift at various temperatures are presented in fig. 2. The temperature dependence of the isomer shift is -7 x mm sec-'OK-'. These data are in good agreement with a recent comprehensive study made of rare earth orthoferrites. l4 Nkel temperatures of Sr,. 6Lao.4Fe03 and Sr,.,La,. ,FeO were determined in a similar fashion and are shown in fig. 3. The N6el temperatures of Sr,.,La,.,FeO and Sro.8Lao.,Fe0 were in this same narrow temperature range while that of Sr,.2Lao.8Fe03 is above room temperature consistent with the high ordering tempera- ture of LaFeO,.Table 2 presents values of the various parameters at room tempera- ture and 78°K for Sr,.,La,.8Fe0 and Sro.8Lao.2Fe03. These materials are not considered in greater detail except to note the distinct oxidation states are resolved 42 SYSTEM Sr,-,La,FeO in the magnetically split samples. The temperature dependence of this resolution for Sr,.8La,.,Fe03 is similar to that for Sr,. ,La,.,FeO which will subsequently be discussed in greater detail. Before proceeding with the results of specimens (x = 0.4-0.6) we consider the phase relations encountered in this system at room temperature. ,- Using X-ray I I I I 740 745 7 50 755 7 60 "K FIG.1.-Determination of the Nkl temperature of LaFeO,. 0 incr. T; 0 decr. T. "K shift mm sec-'. FIG. 2.-Mossbauer parameters of LaFeOs as a function of temperature. 0 Hfs kOe; 0 isomer P. K . GALLAGHER AND J . B. MACCHESNEY 43 diffraction patterns as well as microscopic examinations of pretested specimens we have identified phases generally confirming the earlier results by Shimony et al. A cubic solid solution phase extended from SrFe03 to approximately Sr,. ,La,.4Fe03. An X-ray diffraction pattern of Sr,.,La,.,FeO shows no apparent change in FIG. 3.-Determination of the Nkl temperature of some Srl-xLaxFeO samples a x = 0.4; b x = 06. symmetry at 78°K. A rhombohedral phase exists within narrow limits centred about Sr,.,La,.SFeO,. A narrow range of solid solution was also observed for the orthorhombic perovskite phase whose end member is LaFeO,.Two phase regions separate the regions of solid solution. When considering electronic exchange between iron atoms having different valence states in the samples x = 0.4-0-6 it is important to distinguish between the effects exhibited by the specimens with cubic structure compared to those observed TABLE 2.-MOSSBAUER PARAMETERS FOR Sr,,LaxFe03 WHERE X = 0.2 AND 0.8 iron (IV) iron (111-IV) iron (111) X "K kOe mm sec-1 mm sec-1 kOe mm sec-1 0.2 295 229 - 0 . 4 0 470 +0-13 0.2 78 274 - 0.32 550 + 0-3 1 0.8 295 - 0.06 0.8 78 267 -0.13 395 * *present in too small amount to be determined. for rhombohedral specimens. Values of the isomer shift as a function of temperature for Sro.,La,.,Fe03 are plotted in fig.4. The abrupt discontinuity is apparent at the N6el temperature. Above this temperature the spectra show a single line which has an isomer shift corresponding to an average oxidation state. Repre- sentative spectra are presented in fig. 5. The degree of resolution of the two oxidation 44 0.30 0.20- 0.10 0 % E E \ -0.1 0 - 0'20 - 0.30 I I I - - - \- 0- - - 00 - 0 0 - - O n 00 - I 1 I I 50 I00 I50 2 00 2 50 3 00 I I I I I I I I 1 I I [rn) 0 0 B O 000 a4 8e8 140 160 180 200 220 240 80 channel no. FIG. 5.-Mossbauer spectra of Sro.6T&.4Fe03 at selected temperatures. a 78°K ; Fe(III) 457 kOe +0-23 mm sec-I ; background 1,357,742~ Fe(IV) 259 kOe -0.20 mm sec-I b 187°K; Fe(III) 393 kOe +0.17 mm sec-I background 4,178,104~ - Fe(IV) 237 kOe -0.19 EUII SW-' c 200.7"K ; +0.03 mm sec-l F.W.H.I.0.54 mm sec-l background 376,798~ F.W.H.I. 0.41 mm sec-' ; background 224,268~ d 295°K; -0.01 IIIIII s~c-', P . K . GALLAGHER AND J . B . MACCHESNEY 45 states which would be expected for such a sample can be seen for an analogous sample in fig. 6 of ref. (3). Values of the hyperfine splittings are given in fig. 6. Clearly with Sr,.,Lao.,Fe03 and also for Sro.8La,.,Fe03 the magnetic ordering process gives rise to a marked reduction in the rate of the electron exchange process. Measurements of electrical conductivity of this specimen reflect this process by an abrupt increase in the electrical conductivity upon heating Sr,. 6La,.,Fe03 through its NCel temperatures. Those samples which are rhombohedral behave differently. Fig. 7 shows repre- sentative spectra of Sr,.,Lao.,FeO at various temperatures.Below the N6el temperature two magnetically split spectra are observed corresponding to each oxidation state. Above this temperature the spectra reduce to single broad lines which are also clearly resolved with respect to oxidation state. As the sample approaches room temperature however the electronic exchange rate increases 300 3501 50 I00 I50 2 00 250 "K FIG. 6.-Magnetic hyperfine splitting of Sro. 6Lao.4Fe03 as a function of temperature. so that the two lines eventually become a broad asymmetric smear around 270°K and then begin to narrow into a single line of intermediate isomer shift at room temperature where the electronic exchange time is much less than the lifetimes of the emitting state (1.4 x 10-7sec). It appears that the basic effect in this system involves the temperature coefficient of the rate of electronic exchange Fe+4-0-Fe+3 to Fe+3-O-Fe+4.At room temperature this exchange is rapid in both the cubic and rhombohedral phases. The temperature coefficient is different for the two phases however and the exchange in the rhombohedral phase becomes sufficiently slow to lead to a resolution of oxidation states well before the NCel temperature. The cubic phase would be expected to behave similarly except that the Niel temperature is reached prior to this point of resolution. It appears that the magnetic ordering has a marked effect upon the rate of electronic exchange and in this respect it is interesting to consider a well-known system La,-,CaxMn03.1 Here exchange takes place between Mn3+ and Mn4+ ions having the 3d4 and 3d3 electronic configuration while in the 46 SYSTEM Sr,-,La,FeO present case the iron ions are 3d4 and 3d5.In the Lal-,CaxMn03 system in response to differing concentrations of Mn3f and Mn4+ several different phases are formed having different types of magnetic order. Similar behaviour is expected in this system although such measurements have not yet been completed. It is also valuable to recall that in an antiferromagnet where indirect exchange takes place between magnetic ions i.e. superexchange the spin alignment of adjacent magnetic ions can be antiparallel (G type). If this were the case exchange of c channel no. FIG. 7.-Mossbauer spectra of Sro.5Lao.sFe03 at selected temperatures. a 78°K ; Fe(III) 485 kOe t0.29 mm sec-I ; background 2,227,326~ Fe(IV) 250 kOe - 0.34 mm sec-' background 735 824c Fe(IV) -0.35 mm sec-I ; combined F.W.H.I.0.80 mm sec-' background 215,588~ Fe(IV) -0.37 mm sec-' combined F.W.H.I. 0.80 mm sec-I F.W.H.I. 0.63 mm sec-' ; background 241,864~ F.W.H.I. 0.44 mm sec-I background 2,333,648~ 6 211~7°K; Fe(III) $0.19 mm sec-I c 259°K; Fe(III) f0.15 mm sec-1 ; d 277"K +0.09 mm sec-l e 295°K ; -0.01 mm sec-I electrons between adjacent iron ions would require a change in spin direction. This would be expected to decrease the rate of exchange since at the temperature of magnetic ordering the activation energy for the electronic jump between iron ions would increase abruptly over that due to the simple polarization term because of the addition of an exchange energy term.16* l7 This is not a factor in magnetite because the ions on the B sites are ferromagnetically aligned and consequently rapid electronic exchange is observed below the temperature of magnetic alignment because no change in spin is required.The authors acknowledge the assistance of Messrs. D. N. E. Buchanan for computer programs and J. F. Potter for sample preparation. ' P. K. Gallagher J. B. MacChesney and D. N. E. Buchanan J . Chem. Physics 1964,41 2429. P. K. Gallagher J. B. MacChesney and D. N. E. Buchanan J. Chew. Physics 1965 43 516. P. K. Gallagher J. B. MacChesney and D. N. E. Buchanan J . Chem. Physics 1966,45,2466 J. B. MacChesney J. F. Potter R. C. Sherwood and H. J. Williams J . Chem. Physics 1965 43 1907. P. K. GALLAGHER AND J . B . MACCHESNEY 47 U. Shimony and J. M. Knudsen Physic. Reu. 1966,144,361. R. Bauminger S. G. Cohen A. Marinov S. Ofer and E. Segal Physic. Rev. 1961 122 1447. A. Ito K. Ono and Y. Ishikawa J . Physic. SOC. Japan 1963 18 1465. M. H. Mueller L. Heaton and K. T. Miller Actu Cryst. 1960 13 825. P. K. Gallagher F. Schrey and B. Prescott to be published. ’ K. Ono Y . Ishikawa A. Ito and E. Hirahara J. Physic. SUC. Japan 1962 suppl. Bl 125. l o G. K. Wertheim and R. H. Herber J . Chern. Physics 1963,38,2106. l 2 J. I. Budnick L. J. Bruner R. J. Blume and E. L. Boyd J. Appl. Physics 1961 32 1205. l 3 R. S . Preston S. S. Hanna and T. Heberle Physic. Rev. 1962,128,2207. l4 M. Eibschiitz S. Shtrikman and D. Treves Physic. Rev. 1967 156 562. l5 G. H. Jonker and J. H. Von Santen Physica 1950,16,337. l6 F. J. Morin Physic. Rev. Letters 1959 3 34. l7 D. Adler and H. Brooks Physic. Reu. 1967,155 826.

ISSN:0430-0696    

DOI:10.1039/SF9670100040

出版商:RSC    

年代:1967

数据来源: RSC

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P. K. GALLAGHER AND J . B . MACCHESNEY 47 GENERAL DISCUSSION Prof. N. N. Greenwood (Newcastle upon Tyne) said I was interested in Gallagher's observation of a single line for Fe2+/Fe3+ above the Nkel temperature but the appear- ance of two separate hyperfine fields below it due to the slowing down of electron exchange as a result of antiferromagnetic coupling. Whitfield and I have recently observed a single hyperfine field in cubanite (Cu+Fe2+Fe3+S3) between room tempera- ture and 77°K due to the ferromagnetic exchange interaction between iron ions as in magnetite Fe3+[Fe2+Fe3+]04. Is the occurrence of one or two 6-line spectra in mixed valency iron compounds a general criterion for ferromagnetism on the one hand and antiferromagnetism on the other? Prof. J. F. Duncan (Victoria University of Wellington N.Z.) said Would Gallagher and MacChesney agree that the designation of formal oxidation and valence states as Fe2+ Fe3+ and Fe4+ should not be undertaken without some caution? Whilst it was clear that different isomer shifts were obtained for the species designated in this way the safest conclusion to be drawn was that the electron densities at the 57Fe nucleus were different in these cases.It did not necessarily follow that this was due to successive removal of a single d-electron from the valence shell of the iron. Indeed the absence of quadrupole interactions for Fe4+ suggested that this could not be fairly designated as d4. An alternative possibility is that all five 3d orbitals were occupied for 4/5 of the time by five electrons; or yet again that due to electron delocalization of all five 3d electrons from the iron atom to other parts of the system the eEective screening of the electrons by each of the 3d electrons was only 4/5 of that normally obtained in a 3d5 structure.Many other cases of uncertain or intermehate oxidation states are known of which an exampIe is the nitroprusside ion. This can be written as [Felll(CN)S NO]'- or [Fe"(CN),N0+I3-. Chemically it behaves as iron (11) but the Mossbauer isomer shift 6 suggests iron (111) although it can be fitted on to a linear plot of quadrupole separation AE against 6 for compounds of the type [Fe"(CN),XI3- where X=H20 NH3 CN- NO2 NO+. Spectroscopic (ir.) evidence seems to favour iron (111). Dr. I?. K. Gallagher (Bell Telephone Lab. N.J.) said In reply to Duncan I agree that there are always certain reservations or conditions associated with the assignment and interpretation of oxidation numbers.For the iron compounds which I have 48 GENERAL DI S CUSS10 N discussed however I do not think that it is incorrect to refer to them as compounds of iron(1V). This seems consistent within the general limitations universally applied to such terminology. Admittedly it is a simplified picture but it is just as correct as to say niobium has a + 5 oxidation state in KNb03 or titanium is +4 in the anal- ogous SrTiO compound. One can show that departure from oxygen stoichiometry or La substitution gives rise to the formation of a peak (or peaks) in the Mossbauer spectrum at a position which is consistent with the formation of Fe(II1) at the expense of what I call Fe(1V).The absorption which I attribute to iron(1V) is well outside that of other valence states of iron in oxide systems and is reasonable for that of iron(1V) based on calculations by Danon. There is no indication of any peroxide or super- oxide ion formation based on the crystal structure. The formation of the perovskite structure also suggests a small radius for the iron ion consistent with iron(1V). Prof. J. Danon (Rio de Janeiro) said The values of the isomers shifts listed in table 2 of Gallagher et aZ.’s paper are those expected for the +4 oxidation state of iron according to the modified Walker Wertheim and Jaccarino calibration of the isomer shift of Fe57. Dr. G. M. Bancroft (Ufiiversity of Cambridge) said Could Gallagher comment on the apparent lack of quadrupole splitting in these compounds? Considering the electronic structure of Fe4+ a large quadrupole splitting might be expected.Dr. P. K. Gallagher (Bell Telephone Co. N.J.) said The question of the absence of quadrupole splitting is more interesting. I do agree that the picture of an electron occupying each d orbital four fifths of the time or possibly each dc orbital two thirds of the time is a possible hypothesis; however I do not agree that this does not repre- sent a 3d4 co&pration. This is analogous to the treatment of 3d6[Fe(II)] which does not exhibit quadrupole splitting in a cubic en~ironment.~’~ To date our studies of iron(1V) materials have been restricted to those in which the iron(1v) has cubic symmetry. The exception to this is the Srl-x La,FeO (x<0.5) in our paper. There may well be quadrupole splitting in this material.This is presently under investigation in connection with the determination of the activation energy associated with the electronic exchange in this system. Prof. N. N. Greenwood (Newcastle upon Tym) said Gallagher’s use of the symbol Fe(IV) to indicate an oxidation state of +4 is justified and is more than just a book- keeping convenience. The concept of oxidation state is precisely defined in chemistry and does not necessarily imply an electrostatic charge of +4 on the iron ions. The optical spectra and magnetic properties of Fe(1V) would be quite distinct from those of Fe(II1) for example and the species would contain a different number of unpaired d electrons. Oxidation state should therefore not be confused with electron density on an atom.A particularly clear-cut example of this distinction is the ferricyanide [Fe(CN)J3- and ferrocyanide [Fe(CN),I4- ions. The Mossbauer chemical shift of these two species is almost identical implying identical s electron density at the nucleus and hence identical shielding by the 3d electrons etc. However the oxidation state undoubtedly alters from Fe(III)d5 to Fe(II)d6 as shown by the diamagnetism of the l J. Danon in Applications of the hfossbauer Efect to Chemistry and Solid State Physics (Int. Atomic Energy Agency Vienna Austria) 1966 p. S9. G. K. Wertheim Mossbauer Efect PrincQdes and Application (Academic Press Inc. New York 1964) chap. 6. G. Wertheim €3. Guggenheirn J. Williams and D. Buchanan Physic. Reu. 1967 158,446. F. F. Ham Physic. Rev. 1967 160 328. GENERAL DISCUSSION 49 reduced species.Calculations suggest that although one electron is added to the iron d orbitals approximately 0.9 of an electron is delocalized at ligand-based orbitals via d -pn back donation. The overall electron density around the iron atom is thus simi- lar in the two species despite their differing oxidation states. In this sense Mossbauer chemical shift data provide a useful experimental verification of the old Pauling electroneutrality rule for low spin coordination complexes. It should perhaps be added that alteration of the oxidation state of low spin iron complexes does not always leave the chemical shift unchanged and sometimes the added electron density remains predominantly on metal-based orbitals. For example we have recently shown that reduction of the tris(maleonitriledithio1ate) iron anion from [Fe1vS6C6(CN)6]2- to [Fe111S,C6(CN),]3- increases the chemical shift by 0.15 mmsec-'.Mr. M. G. Clark (University of Cambridge) said In connection with the paper by Gallagher and MacChesney it seems a little surprising that even at low temperatures only small or negligible quadrupole splittings have been observed in Fe(1V) Mossbauer spectra. The simplest picture of the d electrons of high-spin Fe(1V) is a spherically symmetrical 3d5 core containing a positive hole which would be expected to lead to quadrupole splitting. It would be interesting to see how far this simple picture was supported by measurements of the electronic spectrum of Fe(1V). Dr. P. K. Gallagher (Bell Telephone Lab. N.J.) said The iron(1V) compounds which we have measured to date have always provided a cubic environment for the iron(1V).This would explain the absence of the splitting even though the iron is presumably 3d4. A simjlar example is RbFeF,. For the rhombohedral phase Sr0.5La0.5Fe03 (fig. 7 of our paper) there may well be splitting in the iron(1V) spectrum but it is not easily resolved. Dr. A. G. Freeman (Victoria University of Wellington N.Z.) (communicated) The Mossbauer spectra of several graphite-FeCl and graphite-FeCl3-A1Cl3 compounds at room temperature and at 78°K give the following typical results. isomer shift (rel. to natural iron) width at half maximum (mm/sec) (mm/sec) FeC13 (anhydrous) 78" 0.55 f0.04 0.30 f0.05 graphite-FeCl 78" 0.61 &0.04 0.30 d~0-05 graphite-xFeCl3-yA1Cl3 78" 0-62 f0.04 0-30 f0.05 0.1 tx/y<0*25.These observations discount the importance of such bonding species as C,+Cl-FeCl,. 3FeC1 and C:[FeCl4]-FeC1 which have previously been suggested to account for the existence of these compounds. The presence of FeCl would also be indicated by a quadrupole split doublet with rather higher isomer shift than is observed. The shift of the absorption peak to higher velocities is considered significant. It indicated that there has been a general but small increase in the electron density of the d shells of all the iron atoms involved. Presumably these electrons have been donated from the n-electron conductance band of the graphite although donation of graphite n-electrons into the d-shell of thechlorine atoms is probably also ofimportance in bonding the intercalated molecule to the graphite layers.R. G. Shulman and S . Sugano J. Chem. Physics 1965,42 39. T. Birchall N. N. Greenwood and J. A. McCleverty Nature 1967 215 625. 50 GENERAL DISCUSSION Dr. R. Krishnan (Laboratoire de Magn&isme France) said With regard to the paper by Gallagher et al. I wish to make the following remarks concerning the diff- erent valence states of Fe in oxide compounds. In certain cases there are other methods more sensitive than Mossbauer techniques such as ferrimagnetic resonance techniques and determination of thermoelectric properties to detect these valence states of Fe. For example the presence of Fe2+ even in very minute quantities affects pro- foundly the magnetic anisotropy and resonance line width in spinel and garnet ferrites. And lately in Y3Fe5 . OI2 doped with Ca2+ the presence af Fe4+ is indicated by its strong influence on the line width variation with temperature. In the same way elec- trical conductivity and Seebeck coefficient measurements enable a fairly accurate guess of the existing valence states of Fe to be made. The Fe4+(d4) ought to cause Jahn- Teller distortions of the octahedral sites as Mn3+(d4) which is well known for Spinel ferrites and manganites.

ISSN:0430-0696    

DOI:10.1039/SF9670100047

出版商:RSC    

年代:1967

数据来源: RSC

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Calculation of Chemical Shifts in the Mossbauer Spectra of some Tin(1V) Compounds BY N. N. GREENWOOD P. G. PERKINS AND D. H. WALL Dept. of Chemistry The University Newcastle upon Tyne. Received 15th September 1967 The Pople-Segal-Santry SCMO method is used to calculate the electronic structures of a selected series of compounds of Sn(IV) with basis sets including 5s 5p and 5d orbitals on tin. The occupa- tion number of the 5s orbital of tin in each compound is correlated with their known experimental chemical shifts and an approximately linear relationship is found. The 5s electron density at the tin nucleus calculated from the Fenni-Segrk equation using a modified form of Burns’ screening rules also exhibits a linear relation with the observed shifts. The inclusion of 5d orbitals does not appear to exert a marked effect on the calculated s-electron densities.From the results the value of AR/R for the l19Sn nucleus on excitation to the 23.8 keV level is 3 . 5 ~ The differential contribution due to 5s screening of 4s electron density to the total at the nucleus is calculated for the series. Finally the occupation numbers of the three individual 5p orbitals lead to a qualitative understanding of experimental quadrupole splitting data. 1. INTRODUCTION Much discussion has centred on the magnitude and sign of AR/R for the tin nucleus on excitation to the 23.8 keV level and some qualitative and semi-quantita- tive attempts have been made to reconcile Mossbauer data with a particular sign and magnitude of this quantity. Furthermore the chemical shift in the Mossbauer spectrum for a particular compound depends on the s-electron density at the nucleus of the Sn atom.This follows from the eq~ation,~ 2pAR 2 2 2 p 1 3-2p R 0 a r ( 2 p ) 3 + 2 p 6 = n e - R - - 2 -($abs.(0)2 - $em.(o)2). Z The s-electron densities $(0)2 on the right hand side may be written as The 5s electron density does not necessarily correspond to integral numbers of 5s electrons (as would be supposed if we write “ divalent ” tin as Sn2+ and “ quadrivalent ” tin as Sn4+). This is because in tin compounds the bonding between atoms almost always has some covalent character and hence the bond electron density is distributed between the tin atom and the ligand moiety. Moreover the fraction which remains on the tin atom will vary depending on the nature of the ligands e.g. their electro- negativities.This means that if a quantitative relationship between 6 and s-electron density is required and if we are to calculate chemical shifts directly then the problem of calculating the valence electronic structures of tin compounds must be first approached. 51 52 This paper attempts to throw light on these problems by use of a method not previously tried in this context. This is the SCMO method introduced by Pople Santry and Segal which takes account of all valence electrons. This technique has been successfully applied to molecules containing first-row elements of the periodic table in order to obtain information on ground-state properties e.g. electron densities on atoms bond orders between them and electronic energies. In the present treatment it has proved necessary to adapt the method for use in connection with a fourth row element.The salient point is that if the distribution of the s and p electrons of tin bonded in a series of compounds can be calculated then we have a starting point from which the s-electron density at the tin nucleus may be obtained via the Fermi-Segrk expres~ion.~ This should then lead to a value of ARIR. CALCULATION OF Sn(IV) MOSSBAUER SPECTRA 2. THE SELF-CONSISTENT MOLECULAR ORBITAL METHOD The Pople-Segal-Santry SCF method assumes that molecular orbitals for both electrons may be written as linear combinations of orbitals centred on 0 and different atomic sites ; thus The coefficients xLP are calculated by diagonalizing a Fock interaction matrix with elements The quantities HPP HPv are the diagonal and off-diagonal elements of the core Hamiltonian matrix.They are molecular integrals representing the energy of an electron in the field of one or two atoms respectively. The terms YAA YAB have a corresponding function in the electron repulsion matrix whilst the set of Ppv form the spinless density matrix. The set of calculated coefficients xip may then be used to form a new density matrix and the whole process repeated until self-consistency is achieved. The elements of the self-consistent density matrix then yield directly the s and p orbital occupation numbers. ADAPTATION OF THE TECHNIQUE The modifications suggested are similar to those made to the n SCF method by Pariser and Parr and concern the values assigned to the above molecular integrals. (i) The one-centre electron repulsion integrals are approximated according to the formula YAA = IA-AA in which IA and A A are the s orbital valence state ionization potential and electron affinity respectively.The two-centre type are then calculated from these by a series expan~ion.~ (ii) The core Hamiltonian matrix elements are obtained directly or indirectly from the valence state ionization potentials listed by Hinze and Jaffk * rather than by Pople’s technique. Where an orbital initially supplies two electrons I +YAA i s used in place of I:. The off-diagonal elements of the core matrix may N. N. GREENWOOD P. G. PERKINS AND D . H. WALL 53 then be computed from the diagonals via the Mulliken-Wolfsberg-Helmholtz expres- ion,^ i.e. H p Y = q l v ( ~ p + ~ v ) * In selecting compounds to be studied by this method two considerations were of primary importance (a) the compound should in the solid state consist of discrete species of known geometry.This condition makes a study of Sn(I1) systems difficult. (b) The number of different ligands attached to the tin atom was to be restricted in order to minimize the number of parameters in the calculations. The final list of compounds chosen for investigation comprised SnX4 (X = F Cl Br I H Me) SnMe,X (X = F C1 Br I H) SnX,2- (X = F Cl Br I) SnMeH, SnMe,C12 and SnMe,H,. PARAMETERS REQUIRED FOR THE CALCULATIONS The diagonal elements of the core Hamiltonian matrix and of the electron repul- Orbital overlaps required for computing Hpv were sion matrix are given in table 1. TABLE 1 .-VALUES OF MOLECULAR INTEGRALS (eV) H,pW HPc((P) Hwc( cpx) HppW Y A A Sn - 16.16 - 8.32 - - 1.50 8-43 F - 53.26 - 20.86 - 31.98 - 13-87 c1 - 34.78 - 15.03 -22.18 - 9.55 Br -31.31 - 13.10 - 19.15 - 7.56 I - 2546 - 12.67 - 15.45 - 4-43 H - 13.60 - - - 12.84 C - 21 -01 - 11-27 - - 12-1 1 obtained directly from the appropriate “ master formulae ” derived for the purpose.The effective nuclear charges for each atom were estimated by Burns’1o method and the Sn-X bond length in SnX was assumed for all compounds based on tetrahedral symmetry. The Sn-F bond length in SnF,2- is unknown but may be estimated from the radii of the atoms. The two extreme cases i.e. the neutral pair Sn-F and Sn-F- were both examined in the calculations. PARTICIPATION OF ORBITALS OTHER THAN 5s AND 5p A minimum basis set comprising the 5s and 5p valence shells on tin was first considered.The inner filled 4s and 4p levels were not included and were assumed to remain unperturbed except for shielding effects. This approximation is reasonable because these latter atomic levels are considerably more stable than those of the valence shells and so are not expected to enter into bonding to any extent. In order to render the treatment more comprehensive comparable calculations were carried out in which the empty 5d orbitals on tin were also included The inclusion of these orbitals is important because certain of the d-orbitals may mix with the 5s orbital in a way which is governed by the symmetry of the ligand field around the central atom. Furthermore it is important to assess the contribution of d orbitals to the 7~ bonding with the ligands and hence their effect on the electric field gradients.The effective ionization potential of an electron in a 5d-orbital has not previously 54 CALCULATION OF Sn(1V) MOSSBAUER SPECTRA been obtained but it may reasonably be calculated from the known 5p ionization potential and the energy of the process 5s 5p2 +5s 5p5d. Interaction of the tin 6s level with the ligands was tested for stannane. 3. RESULTS ORBITAL OCCUPATION NUMBERS To a first approximation the number of s electrons in the 5s orbital will affect directly the density at the nucleus and so we would expect an approximately linear relationship between the chemical shift and the occupation number of the tin 5s orbital. Table 2 and fig. 1 show the results for the set of compounds calculated with and without inclusion of the 5d orbitals. The reference point for the experi- mental isomer shifts is a-tin.In general a linear relation emerges and as the s-electron orbital density increases so the shift increases. A comparison of the two sets of points of fig. 1 shows that the contribution of d-orbitals to the bonding affects the occupation of the 5s-orbital slightly as also does the inclusion of the 6s orbital in the case of SnH,. The calculated s electron densities for SnI are badly off the line. This is probably due to our neglect of the 5d orbitals on iodine. Inclusion of these excited states is however not practicable in the present calculations. A further important point emerges from this graph. It is often assumed that because of tetrahedral symmetry and ‘‘ sp3 hybridization ” the occupation number of the 5s orbital in a-tin must be exactly one electron.This assumption is theoretically unjustified and the graph indicates a 5s electron population near 1.2 for a-tin. That each Sn atom in a-tin must have exactly four electrons associated with it means that the occupation of each 5p orbital is 3(4-s) and not that the 5s and 5p orbitals each contain exactly one electron. Stannic fluoride has not been considered explicitly because it is not simply tetrahedral in the solid but made up of octahedral units linked by fluorine bridges. It may however be seen that SnF, (6 = -2.5 mm sec-l) would have a 5s occupation of -0.7 electrons. Moreover if it is taken into account that there must also be some 5p occupation in this compound (probably -0.3 electrons per 5p orbital) it is clear that the electronic environment of the tin atom in SnF does not correspond to Sn4+.The latter situation constituted a basic assumption of a previous attempt to calculate ARIR. The present results better satisfy chemical intuition. 4. ESTIMATION OF THE EFFECTIVE ELECTRON DENSITY AT THE TIN NUCLEUS Earlier the chemical shift 6 was correlated directly with the 5s orbital occupation number in the series of compounds. The correct quantity to be used however is the effective s electron density at the nucleus a term explicit in eqn. (1.1). Our knowledge of the occupation numbers of both the 5s and 5p valence orbitals makes direct calculation possible of the effective nuclear charge experienced by any particular s-electron in the outer orbital regions. The Fermi-Segrk equation then allows the relevant s-electron density at the nucleus to be computed ; thus, N.N. GREENWOOD P. G. PERKINS AND D. H. WALL 55 compound SnC14 SnBr4 SnI4 SnH4 SnMe4 SnFMe3 SnClMe3 SnBrMe3 SnIMe3 SnC12Me2 SnHMe3 SnHzMep SnH3Me SnF - (2.2A) SnFi- (2.3 A) SnCla- SnBra- SnTi- TABLE 2.-cHEMICAL SHIFTS AND ELECTRON DENSITIES Electron occupation numbers and densities at the nucleus experimental d mm sec-1 - 1.30~ - 1.00a -0.30~ - 0.83~ - 0.56~ - 0.85u - 0.70~ - 0.65~ - 0.63~ - 0.57e (-0.SSC) - 0.86~ - 0.87~ - 0.86~ - 2 . W (- 2.60b) - 1.60d - 1.23d - 0.85d 5s 0.928 1 0,9874 1.108 (1 * 103)* 1.263 1.146 1.112 1-144 1.015 1.174 1.137 1.120 0.6856 0.7494 0.9105 0.9634 0.9367 - - 5d orbitals included 5d orbitals not included 5p yuSS(O)2 a.u.-3 y4,(0)2 a.u.4 5s 5p ySS(O)2 a u.-3 1.930 11.442 1.941 12-129 2.643 12-872 (2+43) 2.697 14.520 2.264 13.664 2.374 13-174 2.395 13.514 2-145 12.273 2.7 18 13.528 2.692 13.145 2.646 13.003 1-308 8-92 1 1 -406 9.655 1.995 11.177 1.965 11.827 2.152 11.359 - - - - 256.158 256.1 52 256.140 256.125 256.137 256.140 256.1 37 256.150 256.1 34 256.138 256-139 256.183 256.176 256.160 256.155 256.1 58 - - 0.940 1 1.021 1-006 1.108 1.266 1.149 1.115 1-149 1.101 1.176 1.138 1.120 0.6857 0.7600 0.9343 0.9975 0,9882 - 1.996 11.525 2.015 12.458 2-323 12.015 2.659 12.859 2.753 14.493 2.311 13.652 2 - 4 0 13.144 2.460 13.505 2.474 12.955 2.761 13.505 2.727 13.123 2.672 12.979 1-325 8.902 1.419 9.774 2.064 11.401 2.058 12.147 2.332 11.803 - I *6s orbital also included.a M. Cordey-Hayes Applications of the Massbauer Effect in Chemistry and Solid-state Physics Technical Reports b V . I.Gol'danskii E. F. Makarov P. A. Stukan T. N. Sumarokova V. A. Trukhtanov and V. V. Khrapov Dokl. c R. H. Herber and G. I. Parisi Inorg. Chem. 1966 5,769. d N. N Greenwood and J. N. R. Ruddick J. Chem. Soc. A 1967,1679. e R. H. Herber H. A. Stockler and W. T. Reichle J. Chem. Physics 1965 42,2447. Series No. SO,(I.A.E.A. Vienna 1966) p. 156. Akad. Nauk. S.S.S.R. 1964,156,400. 5s orbital occupation number FIG. 1.-Chemical shift plotted against 5s orbital electron occupation. A d-orbitals not included ; 0 d-orbitals included. 56 CALCULATION OF s n ( m MOSSBAUER SPECTRA In this expression 2 is the atomic number Zo is the nuclear charge experienced by any particular s-electron and CT is the quantum defect at the nth level. The factor P is the occupancy of the s orbital under consideration.It is straightforward to apply the equation to evaluate t,hns(0)2 for 12 = 1 2 3 but it may reasonably be assumed that the sum of these densities will remain constant from absorber to emitter and hence will cancel out (see eqn. (1.2)). The electron density in the 4s level will vary to some extent in different cases because of shielding by the 5s electrons. For the present it is also taken as constant in eqn. (1.2). Now the outer valence shells of the tin atom in compounds are incomplete and the problem of screening by a fractional number of electrons arises. In order to deal with this situation we propose a modification of Burns’s screening rules ; thus 2 = Zb(inner shells) - (42) x 0-4 - m x 0.35 (4.2) in which n is the number of 5s electrons and rn the number of 5p electrons.If the derived Zo for each case is now substituted into the Fermi-Segr6 equation the value of t,k5s(0)2 for the tin atom can be calculated. This quantity to first order deter- mines the Mossbauer chemical shift in tin compounds. Table 2 lists the computed densities and fig. 2 illustrates the relation between $5s(0)2 and 6 for the series. The zero of electron density is the constant sum over the inner electron shells and as 0 \ 7 8 9 10 II 12 13 14 15 5s electron density ( a . ~ . ) - ~ FIG. 2.4hemical shift plotted against 5s electron density at the nucleus. (d orbitals included) before the reference point for the chemical shifts is grey tin. The plot is strikingly similar to that of fig. 1 and confirms that there is a linear relation between the s-electron density at the nucleus and the isomer shift.It is perhaps unexpected that fig. 1 and 2 should be so similar in view of the fact that for the first p orbital occupancy is ignored whereas it enters explicitly into the screening factor Zo which yields the second relation. It does however suggest that qualitative predictions of Mossbauer N. N. GREENWOOD P. G. PERKINS AND D . H. WALL 57 chemical shifts in tin(1V) compounds based on intuitive estimates of s orbital occupancy should be reliable. It is now possible to evaluate AR/R from fig. 2 because its slope is equal to A6/A$5s(0)2. Hence AR/R = A6/At,b5s(0)2 x 1.55 x 10-29 eV cm3 = 3.5 x This figure may be compared with the value 1.1 x lo- derived by Boyle Bunbury and Edwards l1 on the supposition that SnF contains Sn4+ and SnCl contains Sn2+ and with the value 1.9 x lo-, calculated by Gol’danskii et a2.l’ The latter resulted from a more realistic appraisal of the electronic structures of tin compounds but was only obtained by recourse to empirical (NQR) data.A value of 3.3 x has also recently been obtained.13 In view of our disagreement with two of these values recalculation of the electron densities at the nucleus using Hartree-Fock wave functions for Sn rather than Burns’ radial functions is in progress. 5. EFFECTS OF SHIELDING OF INNER S-ELECTRONS BY THE 5s ELECTRONS Heretofore it has been assumed that the shells 18-4s are totally unperturbed by bonding and do not make any contribution to the isomer shift. The outer 5s electrons may in fact shield the 1s-4s shells from the nucleus because they penetrate the core up to the nucleus.In covalent compounds the extent of this shielding will depend on the occupation number of the 5s orbital. We can calculate roughly the size of this effect by a modification of the approach of Crawford and Schawlow :14 for a fraction p of the total time a 5s electron is interposed between the ns electron and the nucleus and so reduces the effective nuclear charge Zo on the ns electron by one unit. Hence the fractional change in ns electron density at the nucleus is and so for the fraction of time p t,bns(0)2 is reduced by the fraction 2p/Z0 i.e. it becomes $119(0)2(1 - 2p/Z0). Finally p may be evaluated from the integral (which must be scaled to the occupation number of the 5s orbital) 03 Ti The R-functions are the radial functions for $ns and 1c/5s and so using Burns’ orbitals the integrals can be evaluated in closed form.The correction would be expected to be most important for 4s electrons and indeed it turns out to be small for 4s and negligible for all shells inside this. Accordingly this correction was applied to allow for the screening of the 4s by the 5s electrons and table 2 shows the variation of 4s electron density for the series of compounds. This correction was used when calculating the value of AR/R. The average correction to $4s(0)2 was 0.04 %. 6. QUADRUPOLE SPLITTING AND ELECTRIC FIELD GRADIENTS The hyperfine splitting of a single Mossbauer line results from a non-zero electric field gradient at the nucleus. The diagonal components of the E.F.G. tensor may be related directly to the individual occupation of the three 5p orbitals of the tin atom because these may produce a spherically assymmetric charge distribution around the nucleus.The present treatment is particularly well adapted to throw 58 CALCULATION OF Sn(IV) MOSSBAUER SPECTRA light on this situation because it yields explicitly the occupation numbers of the individual p orbitals. A z z cc M P J - $%(P,) -fn,(py>>(1+ v2/3>+7 (6.1) where q is an asymmetry parameter and Table 3 lists the calculated values of p-orbital imbalance together with the observed quadrupole splittings for the compounds under investigation. The results are not yet particularly quantitative but some points of interest emerge (i) In all cases a molecule with largep-orbital imbalance exhibits a large quadrupole splitting. (ii) Two types of molecule have zero quadrupole splitting ; those with symmetry in which the three p orbitals remain triply degenerate and so are equally populated.This is as expected. Secondly there are those which lack such symmetry but which possess similar occupations of the p orbitals. The latter type of compound repre- sented by SnHMe, SnH,Me should in principle show quadrupole splitting but it seems to be too small to be detected. From the present results the value of p-orbital TABLE 3 .-QUADRUPOLE SPLITTING DATA AND ORBITAL OCCUPATION p-orbital imbalance total 5d average pn-& A nim sec-1 (eqn. (6.1)) occupation bond order Sn-X Sn-Me SnCl SnBr SnI SnH4 SnMe4 SnFMe3 SnC1Me3 SnBrMe SnIMe3 SnC12Me2 SnHMe3 SnH2Me2 SnH3Me SnFg- SnClg- SnBr g- SnIg- 0 0 0 0 0 4.03 3.55 3 a40 3.19 3.41 0 0 0 0 0 0 0 0 0 0 0 0 0.286 0.146 0.148 0.068 0.09 1 0.022 0.007 0.020 0 0 0 0 0.295 0.3 15 0-120 0.07 1 8 0.073 0.130 0.133 0.214 0.050 0-038 0.025 0-125 0-576 0.61 3 1.181 - - 0.0950 0.1060 0-0327 0.0888 0-0989 0.0879 - 0.08 15 0.2005 0.21 95 0,1785 0-0261 0.0300 0,0333 0.0326 0.0265 0.0308 0.0302 0.0292 - imbalance below which no quadrupole splitting will be observed is N 0-03.It is worth- while to examine the electronic structures of certain of these compounds in more detail in order to try to gain some insight into the large differences in the quadrupole effects. Table 3 lists the total occupation of the 5d orbitals and also the total n bond orders between the ligands with filled p or pseudo p orbitals (e.g. F C1 Br Me) and the central atom d orbitals. It is evident that a high quadrupole splitting in a compound can be associated with a high pn-d bond order between ligands and the central atom.This point was made by Gibb and Greenwood l6 and the present calculations appear to confirm their suggestions. The only compound which violates the generalization is SnFMe which exhibits a large quadrupole splitting yet both its 5d occupation number and F-Snp,-d, N. N. GREENWOOD P . G. PERKINS A N D D . H . WALL 59 bond order are small. However whereas for the calculations pyramidal symmetry was assumed for this system it is known that it possesses trigonal bipyramidal units in the solid having the three methyls coplanar with the tin atom and the two fluorines bridging to neighb0urs.l' We thank the General Electric Company for financial support. V. I. Gol'danskii The Mossbauer Effect and its Applications in Chemistry (Consultants Bureau New York 1964) chap.6. I. B. Bersuker V. I. Gol'danskii and E. F. Makarov J. Expt. Theor. Physics 1965,49 699. V . I. Gol'danskii The Mussbauer l2fect and its Applications in Chemistry (Consultants Bureau New York 3964) chap. 3. J. A. Pople D. P. Santry and G. A. Segal J. Chem. Physics 1965 43 S 129. E. Fermi and E. SegrC 2. Physik 1933,82,729. R. Pariser and R. G. Parr J. Chem. Physics 1953 21,466. R. Pariser J. Chem. Physics 1953 21 568. K . Ohno Theor. Chim. Acta 1964,2 219. J. Hinze and H. H. Jaffe J. Amer. Chem. Soc. 1962,84 540. M. Wolfsberg and L. Helmholtz J. Chem. Physics 1952 20 837. Chem. 1952 56,295. R. S. Mulliken J. Physic. l o G. Burns J. Chem. Physics 1964 41 1521. l1 A. J. F. Boyle D. S. Bunbury and C. Edwards Proc. Physic. SOC. 1962 79,416. "V. 1. Gol'danskii G. M. Gorodinskii S. V. Karyagin L. A. Korytko L. M. Krizhanskii E. F. Makarov I. P. Suzdalev and V. V. Khrapov Doklady Akad. Nauk S.S.S.R. 1962 147 127. l3 J. P. Bocquet Y. Y. Chu 0. C. Kistner M. L. Perlman and G. T. Emery Physic. Rev. Letters 1966 17 809. l4 M. F. Crawford and A. L. Schawlow Physic. Rev. 1949,76,1310. l5 T. P. Das and E. L. Hahn Solid State Physics Suppl. 1 1958 chap. 3. 16T. C. Gibb and N. N. Greenwood Applications of the Mossbauer Efect in Chemistry and Solid State Physics Technical Reports Series No. 50 (I.A.E.A. Vienna 1966) p. 163. H. C. Clark R. J. O'Brien and J. Trotter Proc. Chem. SOC. 1963 85.

ISSN:0430-0696    

DOI:10.1039/SF9670100051

出版商:RSC    

年代:1967

数据来源: RSC